wic: Refactored getting root device name

Replaced DirectImageCreator._get_boot_config private method
with a 'rootdev' property.
Simplified the code and API.
Used 'uuid' property instead of incorrectly used 'part_type'.

(From OE-Core rev: 4a303007149ea1205bbd454e70810e7dfa343d4c)

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/lib/wic/plugins/source/rootfs_pcbios_ext.py

@@ -82,7 +82,6 @@ class RootfsPlugin(SourcePlugin):
 
         Called before do_prepare_partition()
         """
-        rootdev = image_creator._get_boot_config()[0]
         options = image_creator.ks.handler.bootloader.appendLine
 
         syslinux_conf = ""
@@ -102,7 +101,7 @@ class RootfsPlugin(SourcePlugin):
         syslinux_conf += "  KERNEL /boot/bzImage\n"
 
         if image_creator.ptable_format in ('msdos', 'gpt'):
-            rootstr = rootdev
+            rootstr = image_creator.rootdev
         else:
             raise ImageError("Unsupported partition table format found")