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>
diff --git a/drivers/reboot-mode/Makefile b/drivers/reboot-mode/Makefile
new file mode 100644
index 0000000..2ab0fdd
--- /dev/null
+++ b/drivers/reboot-mode/Makefile
@@ -0,0 +1,7 @@
+#
+# SPDX-License-Identifier: GPL-2.0+
+#
+# Copyright (c), Vaisala Oyj
+#
+
+obj-$(CONFIG_DM_REBOOT_MODE) += reboot-mode-uclass.o