package/skeleton-init-openrc: add support to remount rootfs as rw

package/skeleton-init-openrc: add support to remount rootfs as rw

Add a pseudo entry for /dev/root in fstab, to tell openrc to properly
remount or not remount / read-write.

For consistency with systemd (which is the only other init system to
tweak an fstab basd on / being rw or not), we do this change in the
openrc skeleton rather than in the openrc package.

Signed-off-by: Michał Łyszczek <michal.lyszczek@bofc.pl>
[yann.morin.1998@free.fr:
  - tweak the sed expressions
  - reword commit log
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>