config: Add 'update_bootimg' command to update flash.bin on Phytec's imx8mm

This command allows easy update on SD card or eMMC of the flash.bin
generated (with binman) during u-boot build.

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Benjamin Hahn <B.Hahn@phytec.de>
Tested-by: Teresa Remmet <t.remmet@phytec.de>
Reviewed-by: Fabio Estevam <festevam@gmail.com>