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
* build: allow compile time - and hence via board config header - choice of boot stage2 via a default compile_time_choice.S boot stage 2 which conditionally includes others
adafruit_rp2040_feather: fix flash size/clkdiv
* fixup GENERIC_03H #define names
* 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
