zedboard: uboot: load env earlier

By default, with the original patch bootcmd runs sdboot before loading uEnv.txt.
Consequently, if the user change modeboot, the user's defined content is updated
too late.
By loading uEnv.txt before 'run $modeboot' the correct boot mode is used instead
of default mode.

Signed-off-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Acked-By: Jan Viktorin <viktorin@rehivetech.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>