Validate ability to have firmware start @ 8MB in NAND

[henrygab]

This relates to ongoing discussions in the buspirate forums about the ability to run alternate firmware on the buspirate, including having one (or more) of those alternate firmwares co-exist in the flash.

To validate this functionality on RP2040, it is necessary to validate that the 2nd stage bootloader can load a firmware that starts at 8MB offset.

Potential steps:

1. Modifying the start address of the main buspirate firmware to offset 8MB.
2. Modify the stage2 bootloader to unconditionally boot from 8MB-offset firmware.
3. Fully erase, then program the above, and verify boot & functionality.
4. Add a second (different) firmware build that starts at standard offset.
5. Adjust stage2 bootloader to conditionally boot from either 8MB-offset firmware, or from alternate firmware at standard offset.
6. Fully erase, then program stage2 bootloader and both firmwares (so they both are available).
7. Verify defaults to boot to 8MB-offset, and that alternate firmware can be rebooted to via software (e.g., with persisted register values indicating to stage2 bootloader...)