[legal] Add a selection of FILE_LICENCE declarations

Add FILE_LICENCE declarations to almost all files that make up the
various standard builds of gPXE.

src/net/tcpip.c

@@ -14,6 +14,8 @@
  * TCP/IP transport-network layer interface
  */
 
+FILE_LICENCE ( GPL2_OR_LATER );
+
 /** Process a received TCP/IP packet
  *
  * @v iobuf		I/O buffer