files: rootfs-postcommands: move helper commands to script

files: rootfs-postcommands: move helper commands to script

OverlayFS systemd helper unit might require more pre-processing
commands. It gets more complicated to embed them in a unit file, because
systemd shell subset is limited and might require additional escaping.
Move the command to a separate script, thus simplifying systemd unit.

Signed-off-by: Vyacheslav Yurkov <v.yurkov@precitec.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>