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