Kconfig: Move DEFAULT_FDT_FILE under boot options

This relates to booting since it is the default devicetree provided to
Linux. Move it under the 'boot options' menu.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/common/Kconfig.boot b/common/Kconfig.boot
index 1696192..4191bfb 100644
--- a/common/Kconfig.boot
+++ b/common/Kconfig.boot
@@ -885,4 +885,9 @@
 	help
 	  This is the default of "preboot" environment variable.
 
+config DEFAULT_FDT_FILE
+	string "Default fdt file"
+	help
+	  This option is used to set the default fdt file to boot OS.
+
 endmenu		# Booting