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/sama5d4ek_mmc_defconfig b/configs/sama5d4ek_mmc_defconfig
index e78460d..a3909a0 100644
--- a/configs/sama5d4ek_mmc_defconfig
+++ b/configs/sama5d4ek_mmc_defconfig
@@ -14,6 +14,7 @@
 CONFIG_DEBUG_UART=y
 CONFIG_FIT=y
 CONFIG_SYS_EXTRA_OPTIONS="SAMA5D4,SYS_USE_MMC"
+CONFIG_ENV_IS_IN_FAT=y
 CONFIG_BOOTDELAY=3
 # CONFIG_CONSOLE_MUX is not set
 CONFIG_SYS_CONSOLE_IS_IN_ENV=y