fall back to MTD if no recovery.fstab is present

Needed for non-device builds (like for the emulator) to work.

Change-Id: I5fdbfb3af34821811744e8ce5bf1eff8df344c73
diff --git a/tools/releasetools/img_from_target_files b/tools/releasetools/img_from_target_files
index 1447b16..7a32621 100755
--- a/tools/releasetools/img_from_target_files
+++ b/tools/releasetools/img_from_target_files
@@ -61,10 +61,11 @@
   img = tempfile.NamedTemporaryFile()
 
   build_command = []
-  if OPTIONS.info_dict["fstab"]["/data"].fs_type.startswith("ext"):
+  fstab = OPTIONS.info_dict["fstab"]
+  if fstab and fstab["/data"].fs_type.startswith("ext"):
     build_command = ["mkuserimg.sh",
                      user_dir, img.name,
-                     OPTIONS.info_dict["fstab"]["/data"].fs_type, "userdata"]
+                     fstab["/data"].fs_type, "userdata"]
     if "userdata_size" in OPTIONS.info_dict:
       build_command.append(str(OPTIONS.info_dict["userdata_size"]))
   else:
@@ -109,10 +110,11 @@
       pass
 
   build_command = []
-  if OPTIONS.info_dict["fstab"]["/system"].fs_type.startswith("ext"):
+  fstab = OPTIONS.info_dict["fstab"]
+  if fstab and fstab["/system"].fs_type.startswith("ext"):
     build_command = ["mkuserimg.sh",
                      os.path.join(OPTIONS.input_tmp, "system"), img.name,
-                     OPTIONS.info_dict["fstab"]["/system"].fs_type, "system"]
+                     fstab["/system"].fs_type, "system"]
     if "system_img" in OPTIONS.info_dict:
       build_command.append(str(OPTIONS.info_dict["system_size"]))
   else: