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/flashable.nix

@@ -14,10 +14,6 @@ in {
     kernel = {
       config = {
         MTD_SPLIT_UIMAGE_FW = "y";
-        # ignore the commandline provided by U-Boot because it's most
-        # likely wrong
-        MIPS_CMDLINE_FROM_BOOTLOADER = lib.mkForce "n";
-        MIPS_CMDLINE_FROM_DTB = "y";
       };
     };