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.

(From OE-Core rev: c3d91b1a2b2ad87c2715a7793f1f408e3b3ba8ef)

Signed-off-by: Adriaan Schmidt <adriaan.schmidt@siemens.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 2fb247c5ecf057bb96649a3c0234794b4991c050)
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/lib/wic/plugins/imager/direct.py

@@ -259,6 +259,8 @@ class DirectPlugin(ImagerPlugin):
             if part.mountpoint == "/":
                 if part.uuid:
                     return "PARTUUID=%s" % part.uuid
+                elif part.label:
+                    return "PARTLABEL=%s" % part.label
                 else:
                     suffix = 'p' if part.disk.startswith('mmcblk') else ''
                     return "/dev/%s%s%-d" % (part.disk, suffix, part.realnum)