How to set level for omxh264enc on TX1 r28.1

How to set the level of h264 file which is encoded by this pipeline

gst-launch-1.0 videotestsrc ! nvvidconv ! omxh264enc profile=1 ! h264parse ! video/x-h264,stream-format=byte-stream ! filesink location=test.h264

The level of the h264 file is baseline@L5.2,How to make it with lower leve?
My TX1 is flashed by Jetpack3.1,r28.1.
Is there some way to set level without update Jetpack?

On r28.1, please apply the patch and rebuild
Source code is at

On r28.2, it can be configured through pipeline:

Thanks you DaneLLL!
It works by rebuild!