Add an option to force usage of the in-kernel cmdline even if the boot
loader passes another command string to the kernel.

Useful if someone cannot or don't want to change the
command-line options of the boot loader but is able to change
the kernel.
Signed-off-by: Alexander Holler <holler@ahsoftware.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>