The correct option for label name in fat and vfat is '-n'

The code in mkfs correctly extends the command line with a '-n' for
vfat but does not currently do it for fat. This means that mkfs for
fat ends up with a '-L' which is what you'd do for everything like
ext[234].

The change just treats fat like vfat in the one place where this check
is required.

Change-Id: If65dfd949acdadff33a564640fb42ea73026a786
Closes-Bug: #1703063

diskimage_builder/block_device/level2/mkfs.py

@@ -110,7 +110,7 @@ class FilesystemNode(NodeBase):
         if self.opts:
             cmd.extend(self.opts)
 
-        if self.type == "vfat":
+        if self.type in ('vfat', 'fat'):
             cmd.extend(["-n", self.label])
         else:
             cmd.extend(["-L", self.label])