* 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
Adds a new cmsis_core INTERFACE library, which provides the basic CMSIS macros/definitions, and RP2040.h and system_RP2040.h
By default the isr_ methods in the vector table are renamed to their CMSIS versions, but this can be overridden by defining PICO_CMSIS_RENAME_EXCEPTIONS=0
* Move vector table to beginning of flash image. Modify a boot2 to use the table.
* Move binary info header up between ELF entry and reset handler
* Extract common routines from boot2_w25q080 into helper files
* Non-default boot2s: use common routines, make callable as void(*)(void)
* BSD3 headers on boot2 files (people might want to use them elsewhere)
* Promote binary info header to its own section, + comment/readability changes
* Fix scope of asminclude path (fixes user custom boot2 definitions)
* Rename symbol, clarify comment
