stage-1-init: add boot.persistence option

This option allows replacing the tmpfs mounted on / by
the live CD's init script with a physical device

Since nixOS symlinks everything there's no trouble
at all.

That enables the user to easily use a nixOS live CD
as a portable installation.

Note that due to some limitations in how the store is mounted
currently only the non-store things are persisted.

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

@@ -144,6 +144,14 @@ for o in $(cat /proc/cmdline); do
             set -- $(IFS==; echo $o)
             stage2Init=$2
             ;;
+        boot.persistence=*)
+            set -- $(IFS==; echo $o)
+            persistence=$2
+            ;;
+        boot.persistence.opt=*)
+            set -- $(IFS==; echo $o)
+            persistence_opt=$2
+            ;;
         boot.trace|debugtrace)
             # Show each command.
             set -x
@@ -534,6 +542,14 @@ while read -u 3 mountPoint; do
       continue
     fi
 
+    if [ "$mountPoint" = / ] && [ "$device" = tmpfs ] && [ ! -z "$persistence" ]; then
+        echo persistence...
+        waitDevice "$persistence"
+        echo enabling persistence...
+        mountFS "$persistence" "$mountPoint" "$persistence_opt" "auto"
+        continue
+    fi
+
     mountFS "$device" "$mountPoint" "$options" "$fsType"
 done