Fix new-keyboard default for RP2040 bootloader (#18100)

lib/python/qmk/constants.py

@@ -20,6 +20,7 @@ VUSB_PROCESSORS = 'atmega32a', 'atmega328p', 'atmega328', 'attiny85'
 
 # Bootloaders of the supported processors
 MCU2BOOTLOADER = {
+    "RP2040": "rp2040",
     "MKL26Z64": "halfkay",
     "MK20DX128": "halfkay",
     "MK20DX256": "halfkay",