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/kzm9g_defconfig b/configs/kzm9g_defconfig
index f1ef1c4..ba75147 100644
--- a/configs/kzm9g_defconfig
+++ b/configs/kzm9g_defconfig
@@ -2,6 +2,7 @@
 CONFIG_ARCH_RMOBILE=y
 CONFIG_TARGET_KZM9G=y
 CONFIG_BOOTDELAY=3
+CONFIG_VERSION_VARIABLE=y
 CONFIG_SYS_PROMPT="KZM-A9-GT# "
 CONFIG_CMD_BOOTZ=y
 CONFIG_CMD_I2C=y