wic: Introduce the --use-label partition parameter

We can use this parameter to make the wic use the label to name a
partition in /etc/fstab.

(From OE-Core rev: 51638edaa00befaed58e2def255d46ae44d9234f)

Signed-off-by: Kevin Hao <kexin.hao@windriver.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

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

@@ -155,6 +155,8 @@ class DirectPlugin(ImagerPlugin):
                         device_name = "UUID=%s" % part.fsuuid
                 else:
                     device_name = "PARTUUID=%s" % part.uuid
+            elif part.use_label:
+                device_name = "LABEL=%s" % part.label
             else:
                 # mmc device partitions are named mmcblk0p1, mmcblk0p2..
                 prefix = 'p' if  part.disk.startswith('mmcblk') else ''