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/am43xx_evm_ethboot_defconfig b/configs/am43xx_evm_ethboot_defconfig
index dfac27b..9892653 100644
--- a/configs/am43xx_evm_ethboot_defconfig
+++ b/configs/am43xx_evm_ethboot_defconfig
@@ -2,6 +2,7 @@
 CONFIG_ARCH_OMAP2PLUS=y
 CONFIG_AM43XX=y
 CONFIG_SYS_EXTRA_OPTIONS="SERIAL1,CONS_INDEX=1,NAND"
+CONFIG_ENV_IS_IN_FAT=y
 CONFIG_SYS_CONSOLE_INFO_QUIET=y
 CONFIG_VERSION_VARIABLE=y
 CONFIG_SPL=y