commands: add of_overlay command for device tree overlays ...

Add a new command "of_overlay" for applying device tree overlays,
because it does not really fit into any other device tree related
command and would create a mess with the command options.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>