[lightning-ln882h] Fix -Wregister warnings from CmBacktrace cmb_def.h

[Bl00d-B0b]

The LN882H SDK CmBacktrace library (cmb_def.h) uses the register storage class specifier in three inline assembly helpers: cmb_get_msp(), cmb_get_psp(), and cmb_get_sp().

This keyword was removed in C++17 (P0001R1), producing spurious -Wregister warnings during every build when compiled with -std=c++17 (as ESPHome does):

cmb_def.h:408:27: warning: ISO C++17 does not allow register storage class specifier [-Wregister]
  408 |         register uint32_t result;

The warning appears 9 times per build (3 inline functions x 3 translation units that include the header).

Fix: Add -Wno-register to CXXFLAGS in builder/family/lightning-ln882h.py. This is a no-op for plain LibreTiny builds (which do not use C++17) and only suppresses the warning for C++ consumers such as ESPHome.

No SDK sources are modified.

[kuba2k2]

Why not just add -Wno-register to GCC compile flags for the entire build?

Side note, LibreTiny doesn't use C++17 by default; ESPHome does.

[Bl00d-B0b]

Agreed, -Wno-register is simpler and cleaner. Happy to rework the PR to use that instead — would just need to revert the sdk_private.h change and add the flag in the right place.

Which build file should it go in? I am thinking somewhere in the lightning-ln882h platform build environment (e.g. a SConscript or platform builder), but I am not familiar enough with the LibreTiny build system to know the canonical location for per-platform compiler flags.

[kuba2k2]

The most logical place to add it would be the file where all other options are defined, builder/family/lightning-ln882h.py. Note that this warning only concerns C++ code, so it should go to the CXXFLAGS variable, instead of the CFLAGS.

[Bl00d-B0b]

Done — reworked the PR to use -Wno-register in CXXFLAGS in builder/family/lightning-ln882h.py as you suggested, and the sdk_private.h pragma change has been reverted.