[legal] Add support for the Unmodified Binary Distribution Licence

[legal] Add support for the Unmodified Binary Distribution Licence

Add the text for the Unmodified Binary Distribution Licence.  This
Licence allows for the distribution of unmodified binaries built from
publicly available source code, without imposing the obligations of
the GNU General Public License upon anyone who chooses to distribute
only the unmodified binaries built from that source code.  See the
licence text for the precise terms and conditions.

Add the licence GPL2_OR_LATER_OR_UBDL to the set of licences which can
be declared using FILE_LICENCE(), and add the corresponding support to
licence.pl.

Signed-off-by: Michael Brown <mcb30@ipxe.org>