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/apf27_defconfig b/configs/apf27_defconfig
index a175274..3f24da9 100644
--- a/configs/apf27_defconfig
+++ b/configs/apf27_defconfig
@@ -2,6 +2,7 @@
 CONFIG_TARGET_APF27=y
 CONFIG_SPL=y
 CONFIG_BOOTDELAY=5
+CONFIG_VERSION_VARIABLE=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="BIOS> "
 # CONFIG_CMD_IMLS is not set