Use general hardening flag toggle lists

The following parameters are now available:

  * hardeningDisable
    To disable specific hardening flags
  * hardeningEnable
    To enable specific hardening flags

Only the cc-wrapper supports this right now, but these may be reused by
other wrappers, builders or setup hooks.

cc-wrapper supports the following flags:

  * fortify
  * stackprotector
  * pie (disabled by default)
  * pic
  * strictoverflow
  * format
  * relro
  * bindnow

pkgs/os-specific/linux/syslinux/default.nix

@@ -16,8 +16,7 @@ stdenv.mkDerivation rec {
   buildInputs = [ libuuid makeWrapper ];
 
   enableParallelBuilding = false; # Fails very rarely with 'No rule to make target: ...'
-  hardening_stackprotector = false;
-  hardening_pic = false;
+  hardeningDisable = [ "pic" "stackprotector" ];
 
   preBuild = ''
     substituteInPlace Makefile --replace /bin/pwd $(type -P pwd)