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/am335x_evm_defconfig b/configs/am335x_evm_defconfig
index 204d817..4ea5617 100644
--- a/configs/am335x_evm_defconfig
+++ b/configs/am335x_evm_defconfig
@@ -7,6 +7,7 @@
 CONFIG_DISTRO_DEFAULTS=y
 CONFIG_SPL_LOAD_FIT=y
 CONFIG_SYS_EXTRA_OPTIONS="NAND"
+CONFIG_ENV_IS_IN_FAT=y
 CONFIG_SYS_CONSOLE_INFO_QUIET=y
 CONFIG_VERSION_VARIABLE=y
 CONFIG_ARCH_MISC_INIT=y