recovery: convert all usages of $prop to ${prop}

Change-Id: I9451b8350d33806097598fb9e575b28fd8b292ad
Signed-off-by: Dima Zavin <dima@android.com>
diff --git a/etc/init.rc b/etc/init.rc
index 554d4e6..9f0ef57 100644
--- a/etc/init.rc
+++ b/etc/init.rc
@@ -38,9 +38,9 @@
     write /sys/class/android_usb/android0/idProduct D001
     write /sys/class/android_usb/android0/functions adb
     write /sys/class/android_usb/android0/enable 1
-    write /sys/class/android_usb/android0/iManufacturer $ro.product.manufacturer
-    write /sys/class/android_usb/android0/iProduct $ro.product.model
-    write /sys/class/android_usb/android0/iSerial $ro.serialno
+    write /sys/class/android_usb/android0/iManufacturer ${ro.product.manufacturer}
+    write /sys/class/android_usb/android0/iProduct ${ro.product.model}
+    write /sys/class/android_usb/android0/iSerial ${ro.serialno}
     start adbd
 
 # Restart adbd so it can run as root