zedboard: Patch default U-Boot environment for zedboard

This patch changes the U-Boot's default environment to
boot Zedboard out-of-the-box from SD card.

The sdboot procedure tries to load a file system.bit into
the Zynq's PL (only if it exists). It is also possible to
alter the booting by an uEnv.txt file located on your SD card.
The uEnv.txt is a plain text file with <key>=<value> pairs
one per line.

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