Support signing target_files without a recovery.fstab.

Change-Id: I7d6f511c69d589bf035ac3eb7eeee4b0e0505faa
diff --git a/tools/releasetools/common.py b/tools/releasetools/common.py
index 2cb5680..4957354 100644
--- a/tools/releasetools/common.py
+++ b/tools/releasetools/common.py
@@ -151,7 +151,8 @@
   try:
     data = zip.read("RECOVERY/RAMDISK/etc/recovery.fstab")
   except KeyError:
-    raise ValueError("Could not find RECOVERY/RAMDISK/etc/recovery.fstab")
+    print "Warning: could not find RECOVERY/RAMDISK/etc/recovery.fstab in %s." % zip
+    data = ""
 
   d = {}
   for line in data.split("\n"):