Fix a small bug when initialize a ota generation.

The os.path.join() will bring in an unwanted backslash.

Test: Tested by starting a new OTA generation process by calling
ota_interface.ota_generate directly.
Test: python test_ota_interface.py -v

Change-Id: I8449eea79303f5aff5188176538eca1291101dff
1 file changed