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/calimain_defconfig b/configs/calimain_defconfig
index 505ab37..9cc4a85 100644
--- a/configs/calimain_defconfig
+++ b/configs/calimain_defconfig
@@ -2,6 +2,7 @@
 CONFIG_ARCH_DAVINCI=y
 CONFIG_TARGET_CALIMAIN=y
 CONFIG_BOOTDELAY=0
+CONFIG_VERSION_VARIABLE=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="Calimain > "
 CONFIG_AUTOBOOT_KEYED=y