Generate UF2 files

[JeremyGrosser]

The examples should use elf2uf2 to build uf2 files that can be flashed with the USB bootloader, for people that don't have a SWD debugger connected. I think we can make gprbuild call elf2uf2 after building the elf.

[Fabien-Chouteau]

Unfortunately with gprbuild you cannot add an extra step after the build.
But with Alire you can: https://alire.ada.dev/docs/ (search for post-build).