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>
diff --git a/env/Kconfig b/env/Kconfig
index 38e7fad..5784136 100644
--- a/env/Kconfig
+++ b/env/Kconfig
@@ -434,6 +434,10 @@
 	                   If none, first valid partition in device D. If no
 	                   partition table then means device D.
 
+	  If ENV_FAT_INTERFACE is set to "mmc" then device 'D' can be omitted,
+	  leaving the string starting with a colon, and the boot device will
+	  be used.
+
 config ENV_FAT_FILE
 	string "Name of the FAT file to use for the environment"
 	depends on ENV_IS_IN_FAT