introduce new feature: BOOTLOADER_LOOPCYCLES_TIMEOUT

This feature must be explicitly defined (Makefile.inc)

When greater than "0", "BOOTLOADER_LOOPCYCLES_TIMEOUT" defines how
many 16bit loopcycles can be cycled, before bootloader times out
and starts user firmware. Of course "BOOTLOADER_CAN_EXIT" must be
enabled.

Even if this value is too small, bootloader  will not exit as
long as bootLoaderConditionSimple() stays true.

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