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/ipam390_defconfig b/configs/ipam390_defconfig
index cee6988..64fa54c 100644
--- a/configs/ipam390_defconfig
+++ b/configs/ipam390_defconfig
@@ -2,6 +2,7 @@
 CONFIG_ARCH_DAVINCI=y
 CONFIG_TARGET_IPAM390=y
 CONFIG_SPL=y
+CONFIG_VERSION_VARIABLE=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="U-Boot > "
 # CONFIG_CMD_IMLS is not set