Before this, CIRCUITPY would start at 1MB anyway. This appeared to work
only because I hadn't checked the actual size of the CIRCUITPY drive,
and because until now the flash hadn't actually crossed that 1MB
boundary into CIRCUITPY storage.
WARNING: on pico_w, upgrading/downgrading CircuitPython across this commit
boundary will erase the CIRCUITPY filesystem. After this commit,
switching between pico and pico_w firmware will erase the CIRCUITPY
filesystem
2bd50673b6