env_mmc: support env partition setup in runtime

Add callback with __weak annotation to allow setup of environment
partition number in runtime from a board file.

Propagate mmc_switch_part() return value into init_mmc_for_env() instead
of -1 in case of failure.

Signed-off-by: Dmitry Lifshitz <lifshitz@compulab.co.il>
Signed-off-by: Igor Grinberg <grinberg@compulab.co.il>
Acked-by: Pantelis Antoniou <panto@antoniou-consulting.com>