Kconfig: Move VERSION_VARIABLE under environment

This relates to the environment so should not be at the top level. Move
it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/env/Kconfig b/env/Kconfig
index f3a9925..c6ba088 100644
--- a/env/Kconfig
+++ b/env/Kconfig
@@ -773,4 +773,13 @@
 
 endif
 
+config VERSION_VARIABLE
+	bool "Add a 'ver' environment variable with the U-Boot version"
+	help
+	  If this variable is defined, an environment variable
+	  named "ver" is created by U-Boot showing the U-Boot
+	  version as printed by the "version" command.
+	  Any change to this variable will be reverted at the
+	  next reset.
+
 endmenu