apalis_imx6: add update_uboot wrapper

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

Usage example:
> tftpboot ${loadaddr} ${board_name}/u-boot.img
> run update_uboot
> tftpboot ${loadaddr} ${board_name}/SPL
> run update_spl

Signed-off-by: Igor Opaniuk <igor.opaniuk@toradex.com>