env/fat.c: allow loading from a FAT partition on the MMC boot device

env/fat.c: allow loading from a FAT partition on the MMC boot device

I don't want to have to specify the device; only the partition.

This allows me to use the same image on internal eMMC or SD card for
Banana Pi R2, and it finds its own environment either way.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
[trini: Add #if/#else/#endif logic around CONFIG_SYS_MMC_ENV_DEV usage,
        whitespace changes]
Signed-off-by: Tom Rini <trini@konsulko.com>