[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/arch/i386/interface/pcbios/ibft.c

@@ -25,6 +25,8 @@
  *
  */
 
+FILE_LICENCE ( BSD2 );
+
 #include <stdint.h>
 #include <stdio.h>
 #include <string.h>