Bootloader: allow altering the startup delay

For boards without a vsense pin, it is necessary to have
sufficient delay to allow enumaration (500 ms). For boards
with a VSENSE pin this can be overridden to a shorter
period.
Sparky: use 2 ms bootloader delay

This allows Spektrum binding. Also, it bumps the bootloader
revision to 0x83.
Quanton: use 2 ms bootloader delay
Colibri: use 2 ms bootloader delay
RevoMini: use 2 ms bootloader delay