phycore-imx8mp: Add overlay and bootenv.txt support

phycore-imx8mp: Add overlay and bootenv.txt support

Add support for loading bootenv.txt as well as loading and applying
overlays during boot from mmc and net.

${no_bootenv}: Prevent loading external bootenv.txt environment. Use
	       ${overlays} variable directly from u-boot environment.
${no_overlay}: Do not load overlays defined in ${overlays} variable.
	       Overlays loaded over the extension command are still
	       being applied.

Signed-off-by: Benjamin Hahn <B.Hahn@phytec.de>
Signed-off-by: Daniel Schultz <d.schultz@phytec.de>
Reviewed-by: Teresa Remmet <t.remmet@phytec.de>
Reviewed-by: Wadim Egorov <w.egorov@phytec.de>