cmake: parse EXTRAFLAGS with separate_arguments

[r3wretrhy]

cmake: parse EXTRAFLAGS with separate_arguments.

Summary

• Why change is necessary: leftover EXTRAFLAGS handling after CMake: Multiple bugfix and optimizations for build system reliability #18194 still splits flags on every space.
• What functional part of the code is being changed: cmake/nuttx_add_library.cmake extra-flag parsing.
• How: use separate_arguments(UNIX_COMMAND) so quoted flags such as -I"/path with spaces" stay one compile option, matching EXTRA_FLAGS.
• Related NuttX Issue: none opened (same class as merged CMake: Multiple bugfix and optimizations for build system reliability #18194).

Impact

• Is new feature added? Is existing feature changed? NO
• Impact on user (will user need to adapt to change)? NO
• Impact on build (will build process change)? YES. EXTRAFLAGS is parsed with shell quoting instead of a naive space split.
• Impact on hardware (will arch(s) / board(s) / driver(s) change)? NO
• Impact on documentation (is update required / provided)? NO
• Impact on security (any sort of implications)? NO
• Impact on compatibility (backward/forward/interoperability)? NO. Unquoted space-separated flags still become one option each.

Testing

I confirm that changes are verified on local setup and works as intended:

• Build Host(s): Windows 11, CMake 3.29.2
• Target(s): cmake -P parse check of EXTRAFLAGS (host CMake, no board image)

Testing logs before change:

EXTRAFLAGS=-Wno-cpp -Werror -I"/tmp/path with spaces/include" string(REPLACE " " ";") OLD_LEN=5 OLD_ITEM=[-Wno-cpp] OLD_ITEM=[-Werror] OLD_ITEM=[-I"/tmp/path] OLD_ITEM=[with] OLD_ITEM=[spaces/include"]

Testing logs after change:

separate_arguments(UNIX_COMMAND) NEW_LEN=3 NEW_ITEM=[-Wno-cpp] NEW_ITEM=[-Werror] NEW_ITEM=[-I/tmp/path with spaces/include] EXTRAFLAGS_PARSE_OK

PR verification Self-Check

• This PR introduces only one functional change.
• I have updated all required description fields above.
• My PR adheres to Contributing Guidelines and Documentation (git commit title and message, coding standard, etc).
• My PR is still work in progress (not ready for review).
• My PR is ready for review and can be safely merged into a codebase.

[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