[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/iscsiboot.c

@@ -13,6 +13,8 @@
 #include <int13.h>
 #include <usr/autoboot.h>
 
+FILE_LICENCE ( GPL2_OR_LATER );
+
 struct setting keep_san_setting __setting = {
 	.name = "keep-san",
 	.description = "Preserve SAN connection",