Adding a parameter 'ttyEmergency'

It specifies what mingetty will be stopped, if a bad filesystem
triggers an emergency shell.

That should be ttyS0 on headless systems, and in that case,
nixos should stop the ttyS0 mingetty from getting in.

modules/tasks/filesystems.nix

@@ -156,6 +156,16 @@ in
       description = "Names of supported filesystem types in the initial ramdisk.";
     };
 
+    boot.ttyEmergency = mkOption {
+      default =
+        if pkgs.stdenv.isArm
+          then "ttyS0" # presumably an embedded platform such as a plug
+        else "tty1";
+      description = ''
+        The tty that will be stopped in case an emergency shell is spawned
+        at boot.
+        '';
+    };
   };
 
 
@@ -247,7 +257,7 @@ in
             status="$(status xserver || true)"
             [[ "$status" =~ start/ ]] && exit 0
 
-            stop tty1 || true
+            stop ${config.boot.ttyEmergency} || true
             
             start --no-wait emergency-shell \
               DEVICE="$DEVICE" MOUNTPOINT="$MOUNTPOINT"