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/omap4_panda_defconfig b/configs/omap4_panda_defconfig
index 3d91f13..22fe337 100644
--- a/configs/omap4_panda_defconfig
+++ b/configs/omap4_panda_defconfig
@@ -4,6 +4,7 @@
 CONFIG_TARGET_OMAP4_PANDA=y
 # CONFIG_SPL_NAND_SUPPORT is not set
 CONFIG_DISTRO_DEFAULTS=y
+CONFIG_ENV_IS_IN_FAT=y
 CONFIG_SYS_CONSOLE_IS_IN_ENV=y
 CONFIG_SYS_CONSOLE_INFO_QUIET=y
 CONFIG_VERSION_VARIABLE=y