Convert labels to upper case

When booting on UEFI, there was an issue mounting the vfat
filesystem. It was caused because the mount was defined in
/etc/fstab in lowercase, but the disk had it labeled in upper
case, and system could not find it. Conver the label to upper
case in case of fat/vfat.

Change-Id: Id3dee735e6f8fb221d199c4aba648f3e9a6e4206

diskimage_builder/block_device/level2/mkfs.py

@@ -58,6 +58,11 @@ class FilesystemNode(NodeBase):
         if self.label is None:
             self.label = self.name
 
+        # for fat/vfat, we use the label as an identifier for the disk
+        # so we need that the label is converted to upper case
+        if self.type in ('vfat', 'fat'):
+            self.label = self.label.upper()
+
         # ensure we don't already have a fs with this label ... they
         # all must be unique.
         if 'fs_labels' in self.state:

diskimage_builder/block_device/tests/test_mount_order.py

@@ -100,7 +100,7 @@ class TestMountOrder(tc.TestGraphGeneration):
             mock.call(['mkfs', '-t', 'xfs', '-L', 'mkfs_var',
                        '-m', 'uuid=var-uuid-1234',
                        '-q', '/dev/loopXp2/var']),
-            mock.call(['mkfs', '-t', 'vfat', '-n', 'varlog',
+            mock.call(['mkfs', '-t', 'vfat', '-n', 'VARLOG',
                        '/dev/loopXp3/var_log'])
         ]
         self.assertEqual(mock_exec_sudo_mkfs.call_count, len(cmd_sequence))