The patch below failed to define the variable, so define it
to make it consistent with M53EVK.

commit a428ac91


Author: Lothar Rubusch <lothar@denx.de>
Date:   Thu Jun 26 11:01:29 2014 +0200

    ARM: m28evk: Update default environment

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Tom Rini <trini@ti.com>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Stefano Babic <sbabic@denx.de>