Mount root filesystem readonly during boot

If creating an image of a disto without grub2 extlinux is instead used,
extlinux was being configured to mount the root filesystem as
read/write which in turn causes e2fsck to fail to test the filesystem,
on RHEL 6.5 this causes the boot process to fail.

Change-Id: I07859e0df9a2bc1ddd8c5f2ed4509ba14312e140
Merge "Mount root filesystem readonly during boot"