unify kernel command line handling

We now use MIPS_CMDLINE_DTB_EXTEND for all boot varieties
(tftpboot, flash boot, kexec) with the addition of
MIPS_BOOTLOADER_CMDLINE_REQUIRE_COOKIE - local patch -
so that the bootloader args are ignored unless they
contain the string "liminix"

modules/base.nix

@@ -79,6 +79,10 @@ in {
         MODULE_SIG = "y";
         DEBUG_FS = "y";
 
+        MIPS_BOOTLOADER_CMDLINE_REQUIRE_COOKIE = "y";
+        MIPS_BOOTLOADER_CMDLINE_COOKIE = "\"liminix\"";
+        MIPS_CMDLINE_DTB_EXTEND = "y";
+
         # basic networking protocols
         NET = "y";
         UNIX = "y";