specify root device as /dev/mtdblockn

the (openwrt?) magic that autodetects it based on mtd labels
won't work with a two-stage boot

modules/base.nix

@@ -119,6 +119,7 @@ in {
     };
     boot.commandLine = [
       "console=ttyS0,115200 panic=10 oops=panic init=/bin/init loglevel=8"
+      "root=${config.hardware.rootDevice}"
       "rootfstype=${config.rootfsType}"
       "fw_devlink=off"
     ];

modules/initramfs.nix

@@ -31,6 +31,11 @@ in
           dir /dev 0755 0 0
           nod /dev/console 0600 0 0 c 5 1
           nod /dev/mtdblock0 0600 0 0 b 31 0
+          nod /dev/mtdblock1 0600 0 0 b 31 1
+          nod /dev/mtdblock2 0600 0 0 b 31 2
+          nod /dev/mtdblock3 0600 0 0 b 31 3
+          nod /dev/mtdblock4 0600 0 0 b 31 4
+          nod /dev/mtdblock5 0600 0 0 b 31 5
           dir /target 0755 0 0
           dir /target/persist 0755 0 0
           dir /target/nix 0755 0 0