Parse included board files for pico_cmake_set declarations

[will-v-pi]

Currently if you have a #include within a board header file (eg in vgaboard.h) any pico_cmake_set and pico_cmake_set_default declarations in the included file aren't parsed (#2112)

This fixes #2112 by parsing those files for these comments. It only parses includes which start with boards/, as it needs to extract the .h file directly for use with pico_find_in_paths.

The inclusion order matches what would be expected (I think) - ie:

// pico_cmake_set PICO_PLATFORM = rp2350

#include "boards/pico.h"

gives PICO_PLATFORM = rp2040 from pico.h,

#include "boards/pico.h"

// pico_cmake_set PICO_PLATFORM = rp2350

gives PICO_PLATFORM = rp2350,

// pico_cmake_set_default PICO_FLASH_SIZE_BYTES = (8 * 1024 * 1024)
#ifndef PICO_FLASH_SIZE_BYTES
#define PICO_FLASH_SIZE_BYTES (8 * 1024 * 1024)
#endif

#include "boards/pico.h"

gives PICO_FLASH_SIZE_BYTES = (8 * 1024 * 1024) (matching the pre-processor defines), and

#include "boards/pico.h"

// pico_cmake_set_default PICO_FLASH_SIZE_BYTES = (8 * 1024 * 1024)
#ifndef PICO_FLASH_SIZE_BYTES
#define PICO_FLASH_SIZE_BYTES (8 * 1024 * 1024)
#endif

gives PICO_FLASH_SIZE_BYTES = (2 * 1024 * 1024) from pico.h (again matching the pre-processor defines)

[lurch]

If the behaviour matches what the pre-processor does, then I think this sounds great! Well done @will-v-pi 😃