imx8mn-ddr4-evk: generate single bootable image

imx8mn-ddr4-evk: generate single bootable image

As suggested in commit 028abfd9b157 ("imx8mm-evk: Generate a single
bootable flash.bin again") for imx8mm_evk, it is possible to produce
single bootable image via binman. This restores the original behavior in
distros, where only one boot container is used to create target image.

Perform similar adaptions in order to provide single bootable image for
imx8mn-ddr4-evk derivate.

Update documentation to drop additional step of copying u-boot.itb

Fixes: 353dfe4b4359 ("imx8mn-ddr4-evk: switch to use binman")
Signed-off-by: Andrey Zhizhikin <andrey.zhizhikin@leica-geosystems.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>