read_only_rootfs_systemd.bbclass: replace rootfs with /dev/root

similar fix has been done in image.bbclass in poky as well;
poky commit: 61b088abb8a1b09ccaf212ab30eddc93c39b55bf

Signed-off-by: Fahad Usman <fahad_usman@mentor.com>
Merge pull request #7 from fahadusman/sb-2096

read_only_rootfs_systemd.bbclass: replace rootfs with /dev/root