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/tools/misc/ipxe/default.nix

@@ -19,8 +19,7 @@ stdenv.mkDerivation {
   preConfigure = "cd src";
 
   # not possible due to assembler code
-  hardening_pic = false;
-  hardening_stackprotector = false;
+  hardeningDisable = [ "pic" "stackprotector" ];
 
   makeFlags =
     [ "ECHO_E_BIN_ECHO=echo" "ECHO_E_BIN_ECHO_E=echo" # No /bin/echo here.