reboot-mode: add support for reboot mode control

A new driver uclass is created to handle the reboot mode control.

The new uclass driver is updating an environment variable with the
configured reboot mode. The mode is extracted from a map provided
at initialization time. The map contains a list of modes
and associated ids.

Signed-off-by: Nandor Han <nandor.han@vaisala.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

drivers/Kconfig

@@ -92,6 +92,8 @@ source "drivers/qe/Kconfig"
 
 source "drivers/ram/Kconfig"
 
+source "drivers/reboot-mode/Kconfig"
+
 source "drivers/remoteproc/Kconfig"
 
 source "drivers/reset/Kconfig"