Fix nasa#2451, minor reorg of message headers

Puts the message encapsulation in a separate header from the message
payload.  Also puts all headers into the "header-check" target as there
were some inclusion dependencies noted.

docs/cFE Application Developers Guide.md

@@ -627,11 +627,11 @@ instead of the default version of the file that is provided from the orignal sou
 
 | **File Name Pattern**      | **Scope** | **Content**                                                                                         |
 |:---------------------------|:---------:|:----------------------------------------------------------------------------------------------------|
-| _module_`_fcncodes.h`      | INTERFACE | Function Codes and associated documentation for the CMD interface of the component (see note)       |
-| _module_`_msgdefs.h`       | INTERFACE | Constant definitions for the CMD/TLM interface(s) of the component (see note)                       |
-| _module_`_tbldefs.h`       | INTERFACE | Constant definitions that affect the table file interface(s) of the component                       |
-| _module_`_msgstruct.h`     | INTERFACE | Structures that define the CMD/TLM message interface(s) of the component                            |
-| _module_`_tblstruct.h`     | INTERFACE | Structures that define the table file interface(s) of the component                                 |
+| _module_`_fcncodes.h`      | INTERFACE | Function Codes and associated documentation for the CMD interface of the component (see note 1)     |
+| _module_`_msgdefs.h`       | INTERFACE | Definitions for the CMD/TLM message payload(s) of the component (see notes 1 & 2)                   |
+| _module_`_tbldefs.h`       | INTERFACE | Definitions for the table file payload(s) of the component (see note 2)                             |
+| _module_`_msgstruct.h`     | INTERFACE | Structures that define the CMD/TLM message interface(s) of the component (see note 2)               |
+| _module_`_tblstruct.h`     | INTERFACE | Structures that define the table file interface(s) of the component (see note 2)                    |
 | _module_`_interface_cfg.h` | INTERFACE | Other configuration that affects the interface(s) of the component (table files and/or messages)    |
 | _module_`_eventids.h`      | INTERFACE | CFE EVS Event IDs for the component, with descriptions/documentation                                |
 | _module_`_global_cfg.h`    |  MISSION  | Constants that need to be consistent across all instances, but do not directly affect interface(s)  |
@@ -644,11 +644,26 @@ instead of the default version of the file that is provided from the orignal sou
 | _module_`_cmds.[ch]`       |    FSW    | Application command processor functions                                                             |
 | _module_`_dispatch.[ch]`   |    FSW    | Dispatch table for validating incoming commands and invoking appropriate command processor          |
 
-**NOTE**: For backward compatibility, the `_msgdefs.h` header should also provide the function code definitions from `_fcncodes.h`, such that
+**NOTE 1**: For backward compatibility, the `_msgdefs.h` header should also provide the function code definitions from `_fcncodes.h`, such that
 inclusion of the `_msgdefs.h` file alone provides the command codes as well as any other required definitions for message interpretation.
 However, the `_fcncodes.h` header should be strictly limited to defining command/function codes for the command interface and should not contain
 any other information.
 
+**NOTE 2**: Table and message definitions are split across a `defs.h` and `struct.h` header file.  The distinction between these two is that the
+`defs.h` file contains those definitions that are coupled to the application source code, whereas the `struct.h` contains those defintions that
+are coupled to the message-passing interface.
+
+Specifically, the `defs.h` file should contain the "Payload" structure definitions - the content of commands that is consumed/produced by the
+application, as well as any constants needed to support those definitions.  In turn, the `struct.h` contains the structures that are actually
+passed via the software bus or messaging interface.  These structures should wrap/encapsulate the message (or table) payload into an object
+that is able to be passed across the interface.  In the case of messages and the software bus interface, this generally means prepending the
+appropriate header object (e.g. `CFE_MSG_CommandHeader_t` or `CFE_MSG_TelemetryHeader_t`) to the message payload object.
+
+The intent of this differentiation is to permit overriding/modification of the encapsulation of messages, without needing to also override
+the content (payload) of those messages.  This supports cases where target system has implemented a custom (non-CFE) encapsulation format
+and all messages and data files are desired to use those formats, as opposed to the normal/default CFE encapsulation formats.  In this case, it
+is important _not_ to change the payload formats, as this will make it more difficult to take a new application update in the future.
+
 **IMPORANT**: All of the header files above with "INTERFACE" scope control the table/message interface of the component.  Changing any of the
 values or definitions in these files will affect the inter-processor communication - either table files, exported data products, commands, or
 telementry messages.  Due caution should be exercised when customizing any of these files, as any changes will need to be propagated to all

modules/es/CMakeLists.txt

@@ -35,6 +35,16 @@ if (ENABLE_UNIT_TESTS)
 endif (ENABLE_UNIT_TESTS)
 
 cfs_app_check_intf(${DEP}
-    cfe_es_msg.h
     cfe_es_eventids.h
-)
+    cfe_es_extern_typedefs.h
+    cfe_es_fcncodes.h
+    cfe_es_interface_cfg.h
+    cfe_es_internal_cfg.h
+    cfe_es_mission_cfg.h
+    cfe_es_msgdefs.h
+    cfe_es_msg.h
+    cfe_es_msgids.h
+    cfe_es_msgstruct.h
+    cfe_es_platform_cfg.h
+    cfe_es_topicids.h
+  )