uboot-imx

Add _end for the end of u-boot image for SMDK6400

Since we rename _end to __bss_end__, But we need add _end symbol for
the end of u-boot image.

Signed-off-by: Zhong Hongbo <bocui107@gmail.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>