[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/syslinux/comboot_resolv.c

@@ -5,6 +5,8 @@
 #include <gpxe/process.h>
 #include <gpxe/resolv.h>
 
+FILE_LICENCE ( GPL2_OR_LATER );
+
 static int comboot_resolv_rc;
 static struct in_addr comboot_resolv_addr;