env: mmc/fat/ext4: make sure that the MMC sub-system is initialized before using it

When booting from a non-MMC device, the MMC sub-system may not be
initialized when the environment is first accessed.
We need to make sure that the MMC sub-system is ready in even a non-MMC
boot case.

Therefore, initialize mmc before loading environment from it.

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>