rootfs-postcommands.bbclass: Make two comments use the new variable syntax

Signed-off-by: Peter Kjellerstedt <peter.kjellerstedt@axis.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit cb17ccca)
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>