stmhal: Activate bootloader with pyb.bootloader()

Signed-off-by: Sven Wegener <sven.wegener@stealer.net>
stmhal: Remap system flash and adjust addresses

Signed-off-by: Sven Wegener <sven.wegener@stealer.net>
Merge pull request #618 from swegener/jump-to-bootloader

Jump to bootloader