mkefidisk: Boot with ro kernel parameter instead of rw

mkefidisk: Boot with ro kernel parameter instead of rw

There is no need to boot with "rw". Booting with "ro" will allow for
fsck to be run during boot, and a proper /etc/fstab will still ensure
the rootfs is "rw" by the time the user can interact with the system.

Change the "rw" to "ro" in the kernel parameters specified in the
generated grub.cfg file.

Fixes [YOCTO 4036] mkefidisk.sh hardcodes 'rw' as root mount option

(From OE-Core rev: 960f0cbf85a4124adbc74d8b2ceb09a7d39ecc04)

Signed-off-by: Darren Hart <dvhart@linux.intel.com>
Cc: Koen Kooi <koen@dominion.thruhere.net>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>