Convert CONFIG_ENV_IS_IN_FAT to Kconfig

This converts the following to Kconfig:
   CONFIG_ENV_IS_IN_FAT

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/hikey_defconfig b/configs/hikey_defconfig
index 88d5853..6df04de 100644
--- a/configs/hikey_defconfig
+++ b/configs/hikey_defconfig
@@ -3,6 +3,7 @@
 CONFIG_IDENT_STRING="hikey"
 CONFIG_DEFAULT_DEVICE_TREE="hi6220-hikey"
 CONFIG_DISTRO_DEFAULTS=y
+CONFIG_ENV_IS_IN_FAT=y
 # CONFIG_DISPLAY_CPUINFO is not set
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_BOARD_EARLY_INIT_F=y