Convert CONFIG_ENV_IS_IN_FLASH to Kconfig

This converts the following to Kconfig:
   CONFIG_ENV_IS_IN_FLASH

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/microblaze/Kconfig b/arch/microblaze/Kconfig
index 80d85e4..20fa25b 100644
--- a/arch/microblaze/Kconfig
+++ b/arch/microblaze/Kconfig
@@ -15,6 +15,7 @@
 	select OF_CONTROL
 	select DM
 	select DM_SERIAL
+	select ENV_IS_IN_FLASH
 
 endchoice