fastboot: annotate switch/case fallthrough case

fastboot: annotate switch/case fallthrough case

The fastboot command handling uses an implicit switch/case fallthrough
when receiving the OEM_CONSOLE command, but when this command is not
enabled in Kconfig, to report this command as unknown.

Add our "fallthrough;" statement-like macro before the default branch in
the fastboot code, to avoid a warning when GCC's -Wimplicit-fallthrough
warning option is enabled.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>