iso-image: add loopback.cfg support to boot iso from grub

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

@@ -183,6 +183,12 @@ for o in $(cat /proc/cmdline); do
         copytoram)
             copytoram=1
             ;;
+        findiso=*)
+            # if an iso name is supplied, try to find the device where
+            # the iso resides on
+            set -- $(IFS==; echo $o)
+            isoPath=$2
+            ;;
     esac
 done
 
@@ -442,6 +448,27 @@ if test -e /sys/power/resume -a -e /sys/power/disk; then
     fi
 fi
 
+# If we have a path to an iso file, find the iso and link it to /dev/root
+if [ -n "$isoPath" ]; then
+  mkdir -p /findiso
+
+  for delay in 5 10; do
+    blkid | while read -r line; do
+      device=$(echo "$line" | sed 's/:.*//')
+      type=$(echo "$line" | sed 's/.*TYPE="\([^"]*\)".*/\1/')
+
+      mount -t "$type" "$device" /findiso
+      if [ -e "/findiso$isoPath" ]; then
+        ln -sf "/findiso$isoPath" /dev/root
+        break 2
+      else
+        umount /findiso
+      fi
+    done
+
+    sleep "$delay"
+  done
+fi
 
 # Try to find and mount the root device.
 mkdir -p $targetRoot