buildroot

zynq_zc706: u-boot: bump to xilinx-v2017.3

[Build-tested only]
Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Acked-by: Alistair Francis <alistair.francis@xilinx.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>