colibri_t20: add update_uboot wrapper

Add universal update_uboot wrapper that helps to update
U-Boot image on internal storage(NAND).

Create an proper tegra image:
$ cbootimage -s tegra20 colibri_t20-512-v12-nand.img.cfg colibri_t20.img

Flash in U-boot shell:
> tftpboot ${loadaddr} ${board_name}/${board_name}.img
> run update_uboot

Signed-off-by: Igor Opaniuk <igor.opaniuk@toradex.com>
Reviewed-by: Oleksandr Suvorov <oleksandr.suvorov@toradex.com>