The first thing I would do is get a baseline with the prebuilt images in petalinux for the 702 board. I would focus on just getting output from u-boot to the console. I would start by booting the images from the SD card, then move to JTAG afterwards.
The basics in XMD that you show are look OK, but it's not clear where the ps7_init.tcl came from and are you building it. You could also start with generating a hello world in the SDK and then making sure you have output to the UART with it working ok. Start simple, then slowly add complexity.