uboot-imx

smdk5420: board: add functions to init dfu environment settings

This commit extends SMDK5420 board's file by adding functions:
- get_dfu_alt_system()
- get_dfu_alt_boot()

This allows setting the DFU environment by function set_dfu_alt_info()
from: board/samsung/common/misc.c

Signed-off-by: Inha Song <ideal.song@samsung.com>
Cc: Akshay Saraswat <akshay.s@samsung.com>
Cc: Minkyu Kang <mk7.kang@samsung.com>