configs: phycore_am64x_a53_defconfig: Fix environment

Enable ENV_OVERWRITE to allow environment variables to be
overwritten within the board code. This is required to add
MAC addresses during SOM detection.

Additionally, set ENV_IS_NOWHERE for boot sources other than MMC.

Signed-off-by: Daniel Schultz <d.schultz@phytec.de>
Reviewed-by: Wadim Egorov <w.egorov@phytec.de>
1 file changed