wic: support rootdev identified by partition label

We already support specifying the rootfs by PARTUUID. This adds general
support for letting the kernel find the rootfs by PARTLABEL.

Signed-off-by: Adriaan Schmidt <adriaan.schmidt@siemens.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 2fb247c5)
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>