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/cairo_defconfig b/configs/cairo_defconfig
index c529c7c..7d8fe11 100644
--- a/configs/cairo_defconfig
+++ b/configs/cairo_defconfig
@@ -3,6 +3,7 @@
 CONFIG_TARGET_OMAP3_CAIRO=y
 CONFIG_SPL=y
 CONFIG_BOOTDELAY=-2
+CONFIG_VERSION_VARIABLE=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="Cairo # "
 CONFIG_CMD_BOOTZ=y