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/bootimg-pcbios.py

@@ -83,7 +83,6 @@ class BootimgPcbiosPlugin(SourcePlugin):
         else:
             splashline = ""
 
-        (rootdev, root_part_uuid) = cr._get_boot_config()
         options = cr.ks.handler.bootloader.appendLine
 
         syslinux_conf = ""
@@ -105,7 +104,7 @@ class BootimgPcbiosPlugin(SourcePlugin):
         syslinux_conf += "KERNEL " + kernel + "\n"
 
         if cr.ptable_format in ('msdos', 'gpt'):
-            rootstr = rootdev
+            rootstr = cr.rootdev
         else:
             raise ImageError("Unsupported partition table format found")