[ambz2] Add elf2bin and flashing support

[afflux]

missing bits:

• [x] check TODOs
• [x] check FIXMEs
• [x] implement flash_write_uf2()
• [ ] test

[afflux]

Flashing is completely untested here. Also, note that I used many features from recent python versions after 3.7:

• int | None instead of typing.Optional[int] (available since 3.10)
• list[int] instead of typing.List[int] (available since 3.9)
• while chunk := f.read(chunk_size): (available since 3.8)
• f"{foo=}" (available since 3.8)

[afflux]

also regarding 49dfddc16347b26c826afa41898ff046532ea570, note that I still use assert, just not for runtime checks. Asserts may not be executed if python is run with -O