[3/10] tools/fdpic: Add the module build helpers the demo apps use.

[casaroli]

Summary

apps/examples/fdpicxip is in apps master and its modules/Makefile reads $(NUTTX_DIR)/tools/fdpic/nuttx-fdpic.mk and $(NUTTX_DIR)/tools/fdpic/fdpic-embed.py, which are not in this tree, so its committed module blobs cannot be regenerated. This adds what that Makefile names.

nuttx-fdpic.mk builds a module: compile with the stock arm-none-eabi GCC, which emits correct FDPIC objects for C and C++, then assemble and link with an arm-uclinuxfdpiceabi binutils, which is the only piece a distribution does not carry. build-binutils.sh builds that, and takes about a minute. nuttx-exports.sh turns a built firmware's exec_symtab.c into a symbol list, fdpic-verify.sh checks a module's imports against it, and fdpic-embed.py turns a module into a C header.

Documentation/components/tools/fdpic.rst describes them under Host Tools, where the tools index picks it up by glob. This answers @cederom's request on #19673 to move the tool README into the documentation.

This is the third of ten PRs that #19673 is being split into, so each can be reviewed on its own. The first four have no FDPIC content at all and do not depend on each other, so they can merge in any order. The remaining six are the FDPIC work itself, one subsystem each, and each is a no-op with CONFIG_FDPIC off: loader core, ARM relocations, the callback entry points, the exec() path, DT_NEEDED through dlopen(), then documentation and a board configuration. The others are [1/10] #19938, [2/10] #19939 and [4/10] #19941.

Impact

Host side only. Nothing in the NuttX build calls these tools, no Kconfig option selects them, and no target code changes.

The loader that runs what they build is proposed separately, so a module built here has nothing in NuttX to load it yet. The page says so.

Testing

make html in Documentation/ produces no warning for the new page. The local build fails on pre-existing duplicate C declarations and a missing plantuml, neither related to this change.

tools/checkpatch.sh passes.

These are the tools that built the module blobs committed in apps/examples/fdpicxip, on macOS with the arm-uclinuxfdpiceabi binutils 2.43 that build-binutils.sh builds.

[github-actions]

MemBrowse Memory Report

No memory changes detected for:

• arduino-mega2560
• esp32-devkitc
• hifive1-revb
• mirtoo
• qemu-armv8a
• qemu-intel64
• rx65n-rsk2mb
• s698pm-dkit
• stm32-nucleo-f103rb