doc: imx: mkimage: reorganize i.MX mkimage documentation

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>
2 files changed