wic: do not use PARTLABEL for msdos partition tables

When using "msdos" partition tables and "--label" but not "--use-uuid"
one can generate images which will not find their root, because
PARTLABEL does not work for "msdos".

Fix that by simply not going the PARTLABEL path in case of "msdos".

Fixes: 2fb247c5ecf0 ("wic: support rootdev identified by partition label")
(From OE-Core rev: 354ef6b723f50b5f0b46a2bf5797e5b982c6ea73)

Signed-off-by: Henning Schild <henning.schild@siemens.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 9ea1a838b946020e026edc032039552b723fcaa4)
Signed-off-by: Steve Sakoman <steve@sakoman.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

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

@@ -259,7 +259,7 @@ class DirectPlugin(ImagerPlugin):
             if part.mountpoint == "/":
                 if part.uuid:
                     return "PARTUUID=%s" % part.uuid
-                elif part.label:
+                elif part.label and self.ptable_format != 'msdos':
                     return "PARTLABEL=%s" % part.label
                 else:
                     suffix = 'p' if part.disk.startswith('mmcblk') else ''