mx51evk: consolidate env for mmcboot and netboot

This patch is to consolidate default mx51evk env for two primary
boot modes, mmcboot and netboot.

It also cleans some unused env like netdev, uboot and redundant
env like loadaddr since CONFIG_LOADADDR already defines it.

Signed-off-by: Shawn Guo <shawn.gsc@gmail.com>