Introduce i.MX8 container set configuration file and add example

Signed-off-by: Peng Fan <peng.fan@nxp.com>

This fixes two small typos in mxsimage.txt.

Signed-off-by: Michael Heimpold <mhei@heimpold.de>
Reviewed-by: Breno Lima <breno.lima@nxp.com>

Signed-off-by: Michael Heimpold <mhei@heimpold.de>
Reviewed-by: Breno Lima <breno.lima@nxp.com>

There is no need to have README in all i.MX documents name.
Remove README from i.MX docs name and add .txt file extension.

Signed-off-by: Breno Lima <breno.lima@nxp.com>
Reviewed-by: Ye Li <ye.li@nxp.com>

The following documents describe the image type used by the mkimage
tool to generate U-Boot images for i.MX devices.

- README.imximage
- README.mxsimage

Move all mkimage related document to doc/imx/mkimage for a better
directory structure.

Signed-off-by: Breno Lima <breno.lima@nxp.com>