releasetools: Fix one missing case for --oem_no_mount.

The CL in [1] added support for --oem_no_mount. It missed one place that
guards the OEM mounting.

[1] commit 8608cde944d64dece63e8c770deb78c1d092c719

Change-Id: I6ecaa94cd0866e0fd4fc88cb0aa3ebf55cde2968
diff --git a/tools/releasetools/ota_from_target_files.py b/tools/releasetools/ota_from_target_files.py
index f51920a..8923a05 100755
--- a/tools/releasetools/ota_from_target_files.py
+++ b/tools/releasetools/ota_from_target_files.py
@@ -1424,7 +1424,8 @@
   if oem_props is not None and len(oem_props) > 0:
     if OPTIONS.oem_source is None:
       raise common.ExternalError("OEM source required for this build")
-    script.Mount("/oem", recovery_mount_options)
+    if not OPTIONS.oem_no_mount:
+      script.Mount("/oem", recovery_mount_options)
     oem_dict = common.LoadDictionaryFromLines(
         open(OPTIONS.oem_source).readlines())