env_mmc: support overriding mmc dev from board code

This enables boards to choose where to/from the environment should be
saved/loaded. They can then for example support using the same device
(dynamically) from which the bootloader was launched to load and save
env data and do not have to define CONFIG_SYS_MMC_ENV_DEV statically.

In my use case, the environment needs to be on the same device I
booted from. It can be the eMMC or an optional SD card.
I therefore would override mmc_get_env_dev in the board code, read the
CPU registers to determine where we booted from and return the
corresponding device index.

Cc: Tom Rini <trini@konsulko.com>
Cc: Stephen Warren <swarren@nvidia.com>
Cc: Tim Harvey <tharvey@gateworks.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Hans de Goede <hdegoede@redhat.com>

Signed-off-by: Clemens Gruber <clemens.gruber@pqgruber.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Tom Rini <trini@konsulko.com>