Merge pull request #618 from swegener/jump-to-bootloader

Jump to bootloader
stmhal: Make pyb.bootloader take no arguments.
stmhal: Fix write_timed function for DAC(2).

Addresses issue #617.
stmhal: Stop USB device before jumping to bootloader

Signed-off-by: Sven Wegener <sven.wegener@stealer.net>