zynq: Fix zynq-boot-bin.py calling for out-of-tree compilation

Fix Makefile to be able to call python script for
out-of-tree compilation.
(make zynq_zc70x_config KBUILD_OUTPUT=<build>)

Reported-by: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
zynq: Fix python version for zynq-boot-bin.py

The code does not work when Python 3 is default.
Enforce using Python 2.

Signed-off-by: Jan Viktorin <jan.viktorin@gmail.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>