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/shmin_defconfig b/configs/shmin_defconfig
index 65b4525..d5f04d8 100644
--- a/configs/shmin_defconfig
+++ b/configs/shmin_defconfig
@@ -1,5 +1,6 @@
 CONFIG_SH=y
 CONFIG_TARGET_SHMIN=y
+CONFIG_VERSION_VARIABLE=y
 # CONFIG_AUTOBOOT is not set
 # CONFIG_CMD_BDI is not set
 # CONFIG_CMD_CONSOLE is not set