update jetson-tx1 documentation

Jetson TX1 image has been update to use Linux For Tegra R28.1

• the device must now be flashed with Jetson TX1 Developer Kit Factory Image (R28.1) http://developer.nvidia.com/embedded/dlc/tx1-production-image before it will work with the b2qt image from sdcard.
• this info should probable be moved from the troubleshooting page to the NVIDIA Jetson TX1 Guide. The previous error message no longer appears, this time the boot just hangs.
• QDB over USB connection now works as in other devices