UF2 Bootloaders for STM32

This implements USB mass storage flashing using UF2 format

Big Notice

You may get the latest compiled bootloader flasher from release page: https://github.com/KittenBot/uf2-meowbit/releases/

And you may go into spi-flash mode by holding the left button during reset.

The internal spi-flash will mount as a thumb disk, format it or copy anything you like to it.

To build just run make in this folder. This will fetch git submodules if needed and build libopenocm3 (which only happens once).

The binaries will be in build/BOARDNAME. The following files will be built:

bootloader.elf - for use with JTAG adapters
bootloader.bin - for direct onboard upgrading
flasher.uf2 - if you already have a UF2 bootloader, you can just drop this on board and it will update the bootloader
flasher16.uf2 - this is similar, but if you have an older UF2 bootloader on the board, that expects the app to start at 16k (0x08004000)

Start from f401 (for 64 pin package) or f401c (for 48 pin package) generic board. Copy the folder and modify board.h, follow instructions at https://arcade.makecode.com/hardware/adding#generating-bootloader

License: LGPL for libopencm3, BSD for what was left of the PX4 bootloader (see LICENSE.md), MIT for the UF2 stuff, and ISC for the bits from dapboot.

It contains small bits from https://github.com/devanlai/dapboot, mostly the WebUSB, that has been heavily modified though.