ARM: imx6: dh-imx6: Add update_sf script to install U-Boot into SF

Add script to read U-Boot from SD card and write it to matching
locations in the SPI NOR, thus making the SPI NOR bootable.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Christoph Niedermaier <cniedermaier@dh-electronics.com>
Cc: Stefano Babic <sbabic@denx.de>
Acked-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>