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/devkit3250_defconfig b/configs/devkit3250_defconfig
index 8ce78be..5e4aefb 100644
--- a/configs/devkit3250_defconfig
+++ b/configs/devkit3250_defconfig
@@ -3,6 +3,7 @@
 CONFIG_SPL=y
 CONFIG_SPL_SYS_MALLOC_SIMPLE=y
 CONFIG_BOOTDELAY=1
+CONFIG_VERSION_VARIABLE=y
 CONFIG_CMD_SPI=y
 CONFIG_CMD_I2C=y
 CONFIG_CMD_USB=y