uboot-imx

M28EVK: Add SD update command

Add "update_sd_firmware" command to easily reload the SD card of
m28evk kit. This comes handy when the board boots from SD card.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Detlev Zundel <dzu@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Wolfgang Denk <wd@denx.de>