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/arch/arm/Kconfig b/arch/arm/Kconfig
index ab469b3..7f6ab4a 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -488,6 +488,7 @@
 	select DM_GPIO
 	select OF_CONTROL
 	imply FAT_WRITE
+	imply ENV_IS_IN_FAT
 
 config TARGET_VEXPRESS_CA15_TC2
 	bool "Support vexpress_ca15_tc2"