env/ext4.c: allow loading from an EXT4 partition on the MMC boot device

This parallels what I added for FAT in commit 6731bef6, allowing the
environment to be found in a specific partition on the device that the
board's mmc_get_env_dev() returns. On the Banana Pi R2 that means the
device that U-Boot was loaded from; either the internal eMMC or an SD
card.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>