fastboot: Add support for 'oem format' command

Introduce 'oem format' which matches the USB implementation, guard this
with CONFIG_FASTBOOT_CMD_OEM_FORMAT so that you can configure it out.

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

include/fastboot.h

@@ -33,6 +33,9 @@ enum {
 	FASTBOOT_COMMAND_REBOOT,
 	FASTBOOT_COMMAND_REBOOT_BOOTLOADER,
 	FASTBOOT_COMMAND_SET_ACTIVE,
+#if CONFIG_IS_ENABLED(FASTBOOT_CMD_OEM_FORMAT)
+	FASTBOOT_COMMAND_OEM_FORMAT,
+#endif
 
 	FASTBOOT_COMMAND_COUNT
 };