arm: mvebu: clearfog: generic distro bootcmd

Switch Clearfog to the generic distro defaults.  This has been taken
from a Debian mailing list thread:

https://lists.debian.org/debian-boot/2016/10/msg00026.html



Signed-off-by: Patrick Wildt <patrick@blueri.se>
Signed-off-by: Stefan Roese <sr@denx.de>