(ubports) init: set property to identify ubports recovery

The UBports installer has no way of knowing, if the recovery currently
used is a UBports specific recovery, which is required in order to
install Ubuntu Touch via system-image-upgrader.

Set a property which we can check for with ADB to detect if a recovery
supporting system-image-upgrader is running.

Issue: ubports#22

Change-Id: I0f2a00a0a8f9b68203804f1c61f9b639cf7227fc
Signed-off-by: Alexander Martinz <amartinz@shiftphones.com>

etc/init.rc

@@ -8,6 +8,9 @@ on early-init
 
     setprop sys.usb.configfs 0
 
+    # Set property to allow UBports installer to identify recovery
+    setprop ro.ubports.recovery true
+
 on init
     export ANDROID_ROOT /system
     export ANDROID_DATA /data