tools/mkexport: kernel mode module/elf flags

[yf13]

Summary

This adds exported LDMODULEFLAGS so that kernel modules can build with Kernel mode NuttX. Without it, we have link errors like:

make[3]: Entering directory '/tmp/apps/examples/module/chardev'
LD:  /tmp/apps/bin/chardev arm-none-eabi-ld: warning: cannot find entry symbol _start; defaulting to 0000000000008000
arm-none-eabi-ld: chardev.c.tmp.apps.examples.module.chardev.o: in function `module_uninitialize':
/tmp/apps/examples/module/chardev/chardev.c:111: undefined reference to `unregister_driver'
arm-none-eabi-ld: chardev.c.tmp.apps.examples.module.chardev.o: in function `module_initialize':
/tmp/apps/examples/module/chardev/chardev.c:129: undefined reference to `register_driver'
arm-none-eabi-ld: /tmp/apps/import/startup/crt0.o: in function `__start':
/tmp/nuttx/arch/arm/src/armv7-a/crt0.c:133: undefined reference to `main'
make[3]: *** [/tmp/apps/Application.mk:308: /tmp/apps/bin/chardev] Error 1

It also stops including NuttX side paths in exported LDELFFLAGS. Currently we have:

$ fgrep LDELFFLAGS apps/import/scripts/Make.defs
LDELFFLAGS = -e main -T /tmp/nuttx/libs/libc/modlib/gnu-elf.ld

But by design, exported SDK shall not refer to NuttX folder any more because apps side may not have access to kernel sources in the SDK scenario.

Impacts

Projects using exported SDK

Testing

• local checks with EXAMPLES_MODULE enabled qemu-armv7a:nsh and qemu-armv7a:knsh
• CI checks

[lupyuen]

Tested OK with rv-virt:knsh64. Thanks :-)
https://gist.github.com/lupyuen/26f74ad08cc11107ba450d51501cf5f2

nsh> uname -a
NuttX 0.0.0 82abf4bc44 Jan 26 2025 16:10:38 risc-v rv-virt
nsh> ostest
ostest_main: Exiting with status 0

[nuttxpr]

[Experimental Bot, please feedback here]

Yes, this PR meets the NuttX requirements.

While the provided summary is good, it could be slightly improved by explicitly mentioning the why. Something like: "This change is necessary to enable building kernel modules with Kernel mode NuttX in exported SDK builds. Currently, missing LDMODULEFLAGS and incorrect LDELFFLAGS cause link errors..."

The Impacts section could be more detailed following the template. Specifically, address each point:

• Is new feature added? Is existing feature changed?: Existing feature (module building) fixed.
• Impact on user: YES. Users building kernel modules with exported SDK will now be able to do so successfully.
• Impact on build: YES. LDMODULEFLAGS are now exported and LDELFFLAGS corrected.
• Impact on hardware: NO.
• Impact on documentation: Potentially YES. If documentation exists regarding building modules with exported SDK, it should be updated. If not, consider adding documentation.
• Impact on security: NO.
• Impact on compatibility: YES. Improves forward compatibility with exported SDK builds. Mention if any backward compatibility concerns exist (unlikely).
• Anything else to consider?: None apparent.

The Testing section is generally adequate but could benefit from more specific details. Instead of "local checks," specify OS, compiler version, NuttX configuration. For example: "Build Host: Linux, GCC 12.2, NuttX configured for qemu-armv7a with Kernel mode and EXAMPLES_MODULE enabled." Similarly, be more explicit with the Target details: "Targets: qemu-armv7a:nsh and qemu-armv7a:knsh."

Including snippets of the actual before/after testing logs (even if short and illustrative) would strengthen the verification claim.