DIB: Add custom block-device-efi element

DIB: Add custom block-device-efi element

Upstream block-device-efi DIB element hardcodes the root partition
filesystem type to 'ext4', hence ignoring our FS_TYPE env var
during image building. Although the image builds successfully,
booting the resulting ISO will run into various issues, especially
since most of our scripts assume an XFS root partition.

To allow overriding the rootfs type, provide our own block-device-efi
DIB element, identical to upstream [1] apart from hardcoding
mkfs.type for /. Our override will take precedence during image
building.

$ diff -Naur \
  ../diskimage-builder/diskimage_builder/elements/block-device-efi/
  ./dib_elements/block-device-efi/
@@ -22,7 +22,6 @@
         type: '8300'
         size: 100%
         mkfs:
-          type: ext4
           mount:
             mount_point: /
             fstab:

[1] https://github.com/openstack/diskimage-builder/tree/2.29.1/ \
    diskimage_builder/elements/block-device-efi

Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>
Change-Id: Ic611bfcf00ad38e8a8a7332a64562f615096207a