Lower the default console log level

It used to be set to 7 (debug) so you get lots of crap on the console.
The new value of 4 is also what Ubuntu uses.  Red Hat uses 3.

A nice side effect is that it's more likely that the LUKS passphrase
prompt doesn't get clobbered by kernel log messages.

modules/system/boot/kernel.nix

@@ -54,6 +54,16 @@ in
       description = "Additional user-defined kernel parameters.";
     };
 
+    boot.consoleLogLevel = mkOption {
+      type = types.int;
+      default = 4;
+      description = ''
+        The kernel console log level.  Only log messages with a
+        priority numerically less than this will appear on the
+        console.
+      '';
+    };
+
     boot.vesa = mkOption {
       default = config.boot.kernelPackages.splashutils != null;
       example = false;
@@ -146,9 +156,14 @@ in
 
     system.modulesTree = [ kernel ] ++ config.boot.extraModulePackages;
 
+    # Implement consoleLogLevel both in early boot and using sysctl
+    # (so you don't need to reboot to have changes take effect).
     boot.kernelParams =
+      [ "loglevel=${toString config.boot.consoleLogLevel}" ] ++
       optionals config.boot.vesa [ "splash=verbose" "vga=0x317" ];
 
+    boot.kernel.sysctl."kernel.printk" = config.boot.consoleLogLevel;
+
     boot.kernelModules = [ "loop" ];
 
     boot.initrd.availableKernelModules =

modules/system/boot/stage-1-init.sh

@@ -66,6 +66,7 @@ mount -t devtmpfs -o "size=@devSize@" none /dev
 mkdir -p /run
 mount -t tmpfs -o "mode=0755,size=@runSize@" none /run
 
+
 # Process the kernel command line.
 export stage2Init=/init
 for o in $(cat /proc/cmdline); do