zedboard: Switch to U-Boot 2015.07

The U-Boot v2015.07 is required for successful out-of-the-box
build for common Xilinx Zynq boards (including Zedboard).

Signed-off-by: Jan Viktorin <viktorin@rehivetech.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>