imx: kontron-sl-mx8mm: Migrate to use CONFIG_EXTRA_ENV_TEXT

Move the environment from the board header to a separate text file
and also drop those variables that are already set in env_default.h
from the Kconfig options.

Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Reviewed-by: Fabio Estevam <festevam@denx.de>
2 files changed