common, kconfig: move VERSION_VARIABLE to Kconfig

move VERSION_VARIABLE from board config file into a
Kconfig option.

Signed-off-by: Heiko Schocher <hs@denx.de>
diff --git a/common/Kconfig b/common/Kconfig
index 4494112..63c4a67 100644
--- a/common/Kconfig
+++ b/common/Kconfig
@@ -203,3 +203,13 @@
 	default n
 	help
 	  This option is used to disable support for parallel NOR flash.
+
+config VERSION_VARIABLE
+	bool "add U-Boot environment variable vers"
+	default n
+	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.