arm: mxs: Define bootscript env variable on m28evk

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

commit a428ac914b
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>

include/configs/m28evk.h

@@ -171,6 +171,7 @@
 #define CONFIG_EXTRA_ENV_SETTINGS					\
 	"consdev=ttyAMA0\0"						\
 	"baudrate=115200\0"						\
+	"bootscript=boot.scr\0"						\
 	"bootdev=/dev/mmcblk0p2\0"					\
 	"rootdev=/dev/mmcblk0p3\0"					\
 	"netdev=eth0\0"							\