[list] Relicense list.h

The code in list.h was originally taken from the Linux kernel many
years ago, but has been rewritten to the point that no original code
remains, and may therefore be relicensed.

The functions and data structures remain largely API-compatible, to
facilitate the conversion of Linux network drivers to iPXE.

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

src/include/ipxe/list.h

@@ -9,7 +9,7 @@
  * list.h.
  */
 
-FILE_LICENCE ( GPL2_ONLY );
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 
 #include <stddef.h>
 #include <assert.h>