[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>

COPYING.UBDL

+UNMODIFIED BINARY DISTRIBUTION LICENCE
+
+
+PREAMBLE
+
+The GNU General Public License provides a legal guarantee that
+software covered by it remains free (in the sense of freedom, not
+price).  It achieves this guarantee by imposing obligations on anyone
+who chooses to distribute the software.
+
+Some of these obligations may be seen as unnecessarily burdensome.  In
+particular, when the source code for the software is already publicly
+and freely available, there is minimal value in imposing upon each
+distributor the obligation to provide the complete source code (or an
+equivalent written offer to provide the complete source code).
+
+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.
+
+The extra permissions granted by this Licence apply only to unmodified
+binaries built from source code which has already been made available
+to the public in accordance with the terms of the GNU General Public
+Licence.  Nothing in this Licence allows for the creation of
+closed-source modified versions of the Program.  Any modified versions
+of the Program are subject to the usual terms and conditions of the
+GNU General Public License.
+
+
+TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
+
+This Licence applies to any Program or other work which contains a
+notice placed by the copyright holder saying it may be distributed
+under the terms of this Unmodified Binary Distribution Licence.  All
+terms used in the text of this Licence are to be interpreted as they
+are used in version 2 of the GNU General Public License as published
+by the Free Software Foundation.
+
+If you have made this Program available to the public in both source
+code and executable form in accordance with the terms of the GNU
+General Public License as published by the Free Software Foundation;
+either version 2 of the License, or (at your option) any later
+version, then you are hereby granted an additional permission to use,
+copy, and distribute the unmodified executable form of this Program
+(the "Unmodified Binary") without restriction, including the right to
+permit persons to whom the Unmodified Binary is furnished to do
+likewise, subject to the following conditions:
+
+- when started running, the Program must display an announcement which
+  includes the details of your existing publication of the Program
+  made in accordance with the terms of the GNU General Public License.
+  For example, the Program could display the URL of the publicly
+  available source code from which the Unmodified Binary was built.
+
+- when exercising your right to grant permissions under this Licence,
+  you do not need to refer directly to the text of this Licence, but
+  you may not grant permissions beyond those granted to you by this
+  Licence.

src/include/compiler.h

 #define FILE_LICENCE_MIT \
 	PROVIDE_SYMBOL ( PREFIX_OBJECT ( __licence__mit__ ) )
 
+/** Declare a file as being under GPLv2+ or UBDL
+ *
+ * This licence declaration is applicable when a file states itself to
+ * be licensed under the GNU GPL; "either version 2 of the License, or
+ * (at your option) any later version" and also states that it may be
+ * distributed under the terms of the Unmodified Binary Distribution
+ * Licence (as given in the file COPYING.UBDL).
+ */
+#define FILE_LICENCE_GPL2_OR_LATER_OR_UBDL \
+	PROVIDE_SYMBOL ( PREFIX_OBJECT ( __licence__gpl2_or_later_or_ubdl__ ) )
+
 /** Declare a particular licence as applying to a file */
 #define FILE_LICENCE( _licence ) FILE_LICENCE_ ## _licence
 

src/util/licence.pl

     desc => "GPL version 2 (or, at your option, any later version)",
     can_subsume => {
       gpl_any => 1,
+      gpl2_or_later_or_ubdl => 1,
       public_domain => 1,
       bsd3 => 1,
       bsd2 => 1,
     can_subsume => {
       gpl_any => 1,
       gpl2_or_later => 1,
+      gpl2_or_later_or_ubdl => 1,
       public_domain => 1,
       bsd3 => 1,
       bsd2 => 1,
       isc  => 1,
     },
   },
+  gpl2_or_later_or_ubdl => {
+    desc => ( "GPL version 2 (or, at your option, any later version) or ".
+	      "Unmodified Binary Distribution Licence" ),
+    can_subsume => {
+      public_domain => 1,
+      bsd3 => 1,
+      bsd2 => 1,
+      mit => 1,
+      isc => 1,
+    },
+  },
   public_domain => {
     desc => "Public Domain",
     can_subsume => {},