introduce new feature: BOOTLOADER_HIDDENEXITCOMMAND

With this "BOOTLOADER_HIDDENEXITCOMMAND" feature, sending the RAW-ISP
command "11111111 xxxxxxxx xxxxxxxx xxxxxxxx" will cause the bootloader
to start the firmware as soon as the programming software disconnects.

Signed-off-by: Stephan Baerwolf <stephan.baerwolf@tu-ilmenau.de>
further trick to optimize code size

This is a very dangerous path to go, but it is worth it.

Signed-off-by: Stephan Baerwolf <stephan.baerwolf@tu-ilmenau.de>