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/sama5d2_xplained_mmc_defconfig b/configs/sama5d2_xplained_mmc_defconfig
index 2602aff..e9a65cc 100644
--- a/configs/sama5d2_xplained_mmc_defconfig
+++ b/configs/sama5d2_xplained_mmc_defconfig
@@ -14,6 +14,7 @@
 CONFIG_DEBUG_UART=y
 CONFIG_FIT=y
 CONFIG_SYS_EXTRA_OPTIONS="SAMA5D2,SYS_USE_MMC"
+CONFIG_ENV_IS_IN_FAT=y
 CONFIG_BOOTDELAY=3
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_SPL=y