Most fastboot commands are suitable for a secure boot environment as
they only allow to download/flash/erase to files/partitions which were
explicitly specified in the usbgadget command.

The "oem" group of commands allows execution of arbitrary barebox
commands. This needs to be disabled for secure boot devices.

Signed-off-by: Christian Eggers <ceggers@arri.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>