[legal] Relicense files under GPL2_OR_LATER_OR_UBDL

These files cannot be automatically relicensed by util/relicense.pl
since they either contain unusual but trivial contributions (such as
the addition of __nonnull function attributes), or contain lines
dating back to the initial git revision (and so require manual
knowledge of the code's origin).

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

src/core/main.c

@@ -12,7 +12,7 @@ Literature dealing with the network protocols:
 
 **************************************************************************/
 
-FILE_LICENCE ( GPL2_OR_LATER );
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 
 #include <stddef.h>
 #include <stdio.h>