Feature request: Add .include directive for pioasm

[thierer]

(I see this was already discussed in https://www.raspberrypi.org/forums/viewtopic.php?f=145&t=313984, but I haven't found an actual issue which was created as a consequence, so here you go)

It would be useful to have an.include directive for pioasm, so .defines relevant for multiple PIO files could be placed in one common file and included from the others.

My current workaround is to use CMake's configure_file(), but an .include directive for pioasm would be more convenient.

[visenri]

Using "pico_generate_pio_header_pre" as I've implemented here:
https://github.com/visenri/pico_cmake_extras
allows the use of all c preprocessor features.

So it can be used to include other files or use preprocessor macros defined elsewhere.

@kilograham:
This should be easy to integrate into the next SDK release.