sunxi: Select environment MMC based on boot device

Currently, the environment is always stored in eMMC if eMMC is enabled
in the config. This means images written to SD and eMMC will cross-
contaminate their environments unless the configuration is changed.

By dropping the device number from the environment location string and
implementing mmc_get_env_dev, we will always use the environment from
the boot device when booting from SD/eMMC.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>