env_mmc: configure environment offsets via device tree

This introduces the ability to override the environment offets from the
device tree by setting the following nodes in '/config':
	'u-boot,mmc-env-offset' - overrides CONFIG_ENV_OFFSET
	'u-boot,mmc-env-offset-redundant'
				- overrides CONFIG_ENV_OFFSET_REDUND

To keep with the previous logic, the CONFIG_* defines still need to
be available and the statically defined values become the defaults,
when the corresponding properties are not set in the device-tree.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Acked-by: Simon Glass <sjg@chromium.org>