build: kconfig + cmake: auto-generated boolean kconfig options for board revisions

[cfriedt]

Is your enhancement proposal related to a problem? Please describe.
Currently, when boards have revisions like bl5340_dvk@1.2.0/nrf5340/cpuapp/ns (ref Board Porting Guide), a Kconfig configuration option CONFIG_BOARD_REVISION="1.2.0" is automatically(?) generated by the build system.

The issue with using a string for this is that the C preprocessor cannot perform conditional preprocessing based on strings.

I..e. the following causes a compilation failure.

#include <stdio.h>

#define CONFIG_BOARD_REVISION "1.2.0"

#if CONFIG_BOARD_REVISION == "1.3.0"
#error only version 1.2.0 is supported
#endif

int main()
{
  return printf("CONFIG_BOARD_REVISION: %s\n", CONFIG_BOARD_REVISION);
}

/tmp/blah.c:5:5: error: invalid token at start of a preprocessor expression
#if CONFIG_BOARD_REVISION == "1.3.0"
    ^
/tmp/blah.c:3:31: note: expanded from macro 'CONFIG_BOARD_REVISION'
#define CONFIG_BOARD_REVISION "1.2.0"
                              ^
1 error generated.

Describe the solution you'd like

Ideally, we would automatically generate a boolean, non-user-configurable, Kconfig option based on the chosen board revision.

Something of the form:

config BOARD_REVISION_1_2_0
    def_bool y

It should be a very simple text translation process:

• convert lowercase characters [a-z] to uppercase [A-Z]
• replace characters outside of [a-zA-Z0-9_] with _
• dynamically generate a file e.g. BOARD_REVISION_CONFIG_IMPLICIT based on CONFIG_BOARD_REVISION and merge the file before BOARD_REVISION_CONFIG into merge_config_files in cmake/modules/kconfig.cmake

The BOARD_REVISION_CONFIG_IMPLICIT file should be optional and should only be generated if there is an active board revision.

This would enable us to do far more things in C like

#ifdef CONFIG_BOARD_REVISION_1_2_0
#endif
#if defined(CONFIG_BOARD_REVISION_1_2_0) || defined(CONFIG_BOARD_REVISION_1_3_0)
#endif

void foo(void)
{
  if (IS_ENABLED(CONFIG_BOARD_REVISION_1_2_0)) {
  } else if (IS_ENABLED(CONFIG_BOARD_REVISION_1_3_0)) {
  } else {
    __ASSERT(false, "not a valid board configuration");
  }
}

Describe alternatives you've considered
Currently, the only solution is to create a series of <revision>_defconfig files and manually add these (I believe), or adding these to Kconfig.defconfig.

E.g.

config BOARD_REVISION_1_2_0
	bool
	default y if "$(BOARD_REVISION)" = "1.2.0"
...

Additional context