Merge patch series "Generalize PHYTEC Overlay Handling"

Daniel Schultz <d.schultz@phytec.de> says:

The overlays are specified in the bootenv.txt file that is loaded into
the environment. Then these overlays get loaded and applied via a script.
These scripts for loading and applying devicetree overlays are identical
for many phytec boards.
Create a common overlays.env that can be included.
Add support for devicetree overlays to phycore-imx8mp and include the
overlays.env for phycore-imx93.
Rename existing environment variables according to bootstd doc.
Move MMC boot logic into a common k3_mmc.env file and include the new
overlays.env file as well.