Add Nucleo F446ZE port to FreeRTOS_apps (#11)

* added nucleo_extension base files WIP

* fixed list of objects in build

* Ping Pong Example Compiles

* Debugging Support, WIP, connecting but not ping ponging

* enabled NVIC on UART3 and added cortex-debug launch file

* Working up to  line 417 in node.c, deallocate(node_secure_root,...)

* added check for null ptr on deallocate

* allocate instead of reallocate if ptr is null, temporary fix?

* remove debug prints

* gitignore update

* cleanup unused wip calls

* moved FreeRTOS-POSIX to lib dir and transport layer to uros_transport dir

* added quick readme to nucleo_f446ze setup

* clean gitignore and vscode debug configuration
Fix STM32 cube MX project


Fix ping_pong app

microros_nucleo_f446ze_extensions/.gitignore

@@ -0,0 +1,4 @@
+.mxproject
+arm_toolchain.cmake
+build/
+core

microros_nucleo_f446ze_extensions/.vscode/launch.json

@@ -0,0 +1,25 @@
+{
+    // Use IntelliSense to learn about possible attributes.
+    // Hover to view descriptions of existing attributes.
+    // For more information, visit: https://go.microsoft.com/fwlink/?linkid=830387
+    "version": "0.2.0",
+    "configurations": [
+
+        {
+            "type": "cortex-debug",
+            "servertype": "openocd",
+            "request": "launch",
+            "name": "Debug microROS",
+            "executable": "${workspaceRoot}/build/micro-ROS.elf",
+            "rtos": "FreeRTOS",
+            "device": "STM32F446ZE",
+            "armToolchainPath": "${workspaceRoot}/../../toolchain/bin/",
+            "configFiles": [
+                "interface/stlink-v2-1.cfg",
+                "target/stm32f4x.cfg",
+            ],
+            "cwd": "${workspaceRoot}",
+            "svdFile": "./STM32F446.svd"
+        }
+    ]
+}