CF2 support

[Michał Moskal]

Describe the solution you'd like

UF2 defines a format to store configuration data about the board in the bootloader area, see https://github.com/microsoft/uf2/blob/master/cf2.md

This is currently used primarily by MakeCode Arcade to define where the button/screen pins are, but is extensible. This is supported in SAMD51 and STM32F4 bootloaders.

Could we have maybe 1k of bootloader area reserved for this?

[Michał Moskal]

CC @ladyada @dhalbert

[Ha Thach]

This is a great idea, 1KB is not a problem or whatsover, we does reserve several of KBs for future extensions as this