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This change allows to build firmware for different rp2-based boards,
following how it is done in other ports like stm32 and esp32.  So far only
the original Pico and Adafruit Feather RP2040 are added.  Board names
should match (sans case) those in pico-sdk/src/boards/include/boards/.

Usage: Pico firmware can be build either using make as previously (it is
the default board) or by `make BOARD=PICO`.  Feather is built by `make
BOARD=ADAFRUIT_FEATHER_RP2040`.  Only the board name and flash drive size
is set, pin definition is taken from the appropriate pico-sdk board
definition.  Firmware is saved in the directory build-BOARD_NAME.