Call bootloader when main exits.

Make the development cycle much shorter. Instead of messing with the
BOOT0 jumper and having to reset the board manually, it can all be done
from the computer now:

* Send a well-known sequence to the main app to terminate it
* Then run stm32flash as usual

No need to put a boot0 jumper, or press the reset button, or anything.
Implement simple UART reading and enable app exit.

App exit is done using the 'U' (0x55) character for now.