This adds `make flash` support for the rp2040 target. Flashing is
performed using a custom `rp2040_flash` tool that uses the PICOBOOT
protocol. Root is not required.
The user specifies the serial device of the rp2040 they wish to flash as
the device. This device is reset into bootsel mode and `rp2040_flash`
is invoked on the original USB device path.
If the device is already in bootloader mode, the user can specify
'first' as `FLASH_DEVICE` which will simply invoke `rp2040_flash` with
no bus/address options.
Signed-off-by: Lasse Dalegaard <dalegaard@gmail.com>
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