boot_serial: Allow platforms to catch any unprocessed groups
boot_serial: Allow platforms to catch any unprocessed groups

The commit modifies mcumgr group processing in boot_serial_input
to allow catching any group that has not been processed by user
provided function.

Signed-off-by: Dominik Ermel <1a1d45a9cc0c98a37f8d0a0d2dbe3cacc0b2344f@nordicsemi.no>

ATmobica/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,runtimeco/mcuboot