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
1 file changed