The SPL_SAVEENV functionality, when working with an MMC device, can only
work with SPL_MMC_WRITE enabled. This however only works with SPL_MMC
also being enabled. Update the dependencies to show that if we have
enabled SPL_ENV_IS_IN_MMC then we select SPL_MMC_WRITE and make
SPL_ENV_IS_IN_MMC depends on SPL_MMC.
Signed-off-by: Tom Rini <trini@konsulko.com>