[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/tcp.c

@@ -21,6 +21,8 @@
  *
  */
 
+FILE_LICENCE ( GPL2_OR_LATER );
+
 /** A TCP connection */
 struct tcp_connection {
 	/** Reference counter */