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/devkit8000_defconfig b/configs/devkit8000_defconfig
index c8021c6..29a246c 100644
--- a/configs/devkit8000_defconfig
+++ b/configs/devkit8000_defconfig
@@ -2,6 +2,7 @@
 CONFIG_OMAP34XX=y
 CONFIG_TARGET_DEVKIT8000=y
 CONFIG_SPL=y
+CONFIG_VERSION_VARIABLE=y
 CONFIG_HUSH_PARSER=y
 # CONFIG_CMD_IMI is not set
 # CONFIG_CMD_IMLS is not set