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/hsdk_defconfig b/configs/hsdk_defconfig
index 0018c04..eb3dfbd 100644
--- a/configs/hsdk_defconfig
+++ b/configs/hsdk_defconfig
@@ -4,6 +4,7 @@
 CONFIG_SYS_TEXT_BASE=0x81000000
 CONFIG_SYS_CLK_FREQ=1000000000
 CONFIG_DEFAULT_DEVICE_TREE="hsdk"
+CONFIG_ENV_IS_IN_FAT=y
 CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_SYS_PROMPT="hsdk# "
 # CONFIG_CMD_IMLS is not set