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/configs/walnut_defconfig b/configs/walnut_defconfig
index 5f7b40d..f927c4c 100644
--- a/configs/walnut_defconfig
+++ b/configs/walnut_defconfig
@@ -3,6 +3,7 @@
 CONFIG_TARGET_WALNUT=y
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_BOOTDELAY=5
+CONFIG_VERSION_VARIABLE=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_ASKENV=y
 CONFIG_CMD_GREPENV=y