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/omap3_zoom1_defconfig b/configs/omap3_zoom1_defconfig
index db2aafc..e92d9c6 100644
--- a/configs/omap3_zoom1_defconfig
+++ b/configs/omap3_zoom1_defconfig
@@ -1,6 +1,7 @@
 CONFIG_ARM=y
 CONFIG_OMAP34XX=y
 CONFIG_TARGET_OMAP3_ZOOM1=y
+CONFIG_VERSION_VARIABLE=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMI is not set