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/k2g_evm_defconfig b/configs/k2g_evm_defconfig
index 1eed605..332c4d1 100644
--- a/configs/k2g_evm_defconfig
+++ b/configs/k2g_evm_defconfig
@@ -10,6 +10,7 @@
 CONFIG_SPL_SPI_SUPPORT=y
 CONFIG_DEFAULT_DEVICE_TREE="keystone-k2g-evm"
 CONFIG_OF_BOARD_SETUP=y
+CONFIG_ENV_IS_IN_FAT=y
 CONFIG_SYS_CONSOLE_INFO_QUIET=y
 CONFIG_DTB_RESELECT=y
 CONFIG_FIT_EMBED=y