[ot] scripts/opentitan: add a small GDB replay script from QEMU exec log

docs/opentitan/gdbreplay.md

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+# `gdbreplay.py`
+
+`gdbreplay.py` uses QEMU log files containing `exec` messages to help a GDB client to replay
+execution of a guest application.
+
+It parses QEMU log files, and creates a GDB server serving GDB remote requests. GDB client can
+add breakpoints and follow executed instructions by the guest, stepping or running till a
+breakpoint is encountered. It is possible to run execution backward.
+
+Supported GDB commands:
+
+ * step/step instruction
+ * reverse step/step instruction
+ * continue
+ * hardware breakpoint
+ * memory dump (code only), requires to provide the application(s) with the help of ELF or binary
+   files
+
+QEMU traces do not contain register values so it is not possible to observe any register content or
+data.
+
+## Usage
+
+````text
+usage: gdbreplay.py [-h] [-t LOG] [-e ELF] [-a ADDRESS] [-b BIN] [-g GDB] [-v] [-d]
+
+QEMU GDB replay.
+
+options:
+  -h, --help            show this help message and exit
+  -t LOG, --trace LOG   QEMU execution trace log
+  -e ELF, --elf ELF     ELF application
+  -a ADDRESS, --address ADDRESS
+                        Address to load each specified binary
+  -b BIN, --bin BIN     Binary application
+  -g GDB, --gdb GDB     GDB server (default to localhost:3333)
+  -v, --verbose         increase verbosity
+  -d, --debug           enable debug mode
+````
+
+### Arguments
+
+* `-a` specify an address where to load the matching RAW binary application, see `-b` option for
+  details. May be repeated.
+
+* `-d` only useful to debug the script, reports any Python traceback to the standard error stream.
+
+* `-b` specify a RAW binary application. May be repeated. For each `-b` argument, there should be
+  a matching `-a` argument.
+
+* `-e` specify a ELF application to load. May be repeated.
+
+* `-g` specify an alternative interface/port to listen for GDB client
+
+* `-t` specify the QEMU log to parse for `exec`ution traces (see `-d exec` QEMU option)
+
+* `-v` can be repeated to increase verbosity of the script, mostly for debug purpose.
+
+### Examples
+
+* From one terminal, starts the GDB replay tool
+  ````sh
+  # Load two ELFs here, one for the ROM, one for the application to debug
+  ./scripts/opentitan/gdbreplay.py -vv -t qemu.log \
+      -e test_rom_fpga_cw310.elf -e csrng_kat_test_prog_fpga_cw310.elf
+  ````
+
+* From another terminal, starts a regular GDB client
+  ````sh
+  riscv64-unknown-elf-gdb
+  ````
+  then
+  ````
+  # Load the ELF file on GDB side so that it knows the symbols
+  file csrng_kat_test_prog_fpga_cw310.elf
+  # Connect to gdbreplay
+  target remote :3333
+  # Add a HW breakpoint to the entry function of the test
+  hb test_main
+  # Execute till breakpoint
+  c
+  ...
+  # Move execution back to the `test_main` caller
+  rsi
+  # Move execution point to the very first instruction
+  rc
+  ````