imx8m: Switch to fsl mkimage, don't use precompiled firmware (DDR, atf, ...) files anymore.

Prior to this we used firmware files used by imx-boot that have been precompiled checked in inside the mkimage repo. The intended freescale way is to use the imx-atf and imx-firmware-8m recipes to download and build these. This is what we are doing now. It also allows changes to the imx-atf code, for example. We also switch to the freescale mkimage repo, as we don't need the precompiled files anymore.