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/axs103_defconfig b/configs/axs103_defconfig
index add24cf..ff0bf19 100644
--- a/configs/axs103_defconfig
+++ b/configs/axs103_defconfig
@@ -3,6 +3,7 @@
 CONFIG_SYS_TEXT_BASE=0x81000000
 CONFIG_SYS_CLK_FREQ=100000000
 CONFIG_DEFAULT_DEVICE_TREE="axs103"
+CONFIG_ENV_IS_IN_FAT=y
 CONFIG_BOOTDELAY=3
 CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_SYS_PROMPT="AXS# "