wic/plugins/images/direct: Allow changes in fstab on rootfs

wic/plugins/images/direct: Allow changes in fstab on rootfs

Allow wic to also manipulate the rootfs entry in fstab, which it
currently refuses to write. Reasons one might want to do that include
using systemd-growfs via --fsoptions on /
With this change / is now handled exactly the same as other
mountpoints, the former exception seemingly was not even documented.

Signed-off-by: Tobias Schmidl <tobiasschmidl@siemens.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>