* Platform updates
- Add PICO_RP2040=1 to rp2040 builds
- Add new PICO_RP2040_B0/1/2_SUPPORTED macros and retailer chip specific code to use
- Add doxygen to platform.h
- Make pico.h includable from assembly (because header order is important and tricky) - split out platform_asm.h
- Switch to using PICO_RP2040_B0_SUPPORTED in board headers
In C, func() is a function taking an unspecified number of arguments,
vs func(void) a function taking no arguments. In C++ both forms indicate
"no arguments."
Update these headers to use the (void) form, which is correct in both
languages and avoids complaints when -Wstrict-prototypes is specified.
