Improve support for hand-written asm source files

[ivankravets]

Improve support for hand-written asm source files and document flag scopes

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[me-no-dev]

these parts of the files are automatically generated by the lib-builder. Values are extracted from IDF's build system.

[ivankravets]

How about with ASFLAGS and ASPPFLAGS? There is a bug in the current scripts with "-x", "assembler-with-cpp" which is set for ASFLAGS instead of ASPPFLAGS.

[me-no-dev]

This is the part of the script that generates your configs. Above it is how they are obtained. I suggest you look there for resolution :)

[ivankravets]

Could you adapt my fixes for the ASM? I'm not so good in the Bash. I'll close this PR. Thanks.

[me-no-dev]

will do my best :)

[me-no-dev]

@ivankravets will the change for "ASPPFLAGS" be enough? Can I just rename "ASFLAGS" to "ASPPFLAGS"?

[ivankravets]

If you don't use bare ASM (without C preprocessor, small .s), then just rename ASFLAGS -> ASPPFLAGS.

The ASFLAGS are common for .s and .S, whereas ASPPFLAGS only for .S. The .s very often require machine-related flags. However, it seems they are not required for ESP toolchains.

[me-no-dev]

Will be part of 2.0.4 :)