Add support for flashing ROM separately with uf2 or picotool

[kbeckmann]

Use case:

• User downloads base firmware from github action artifact, flashes using UF2 mass storage
• User compresses ROM with a tool (python or online javascript thing), flashes it using picotool or UF2 mass storage

Let's start with the picotool approach, then build a javascript online tool that generates a uf2 file with a compressed ROM.