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/bcm23550_w1d_defconfig b/configs/bcm23550_w1d_defconfig
index ca4feaf..579a2ba 100644
--- a/configs/bcm23550_w1d_defconfig
+++ b/configs/bcm23550_w1d_defconfig
@@ -1,6 +1,7 @@
 CONFIG_ARM=y
 CONFIG_ENABLE_ARM_SOC_BOOT0_HOOK=y
 CONFIG_TARGET_BCM23550_W1D=y
+CONFIG_VERSION_VARIABLE=y
 CONFIG_HUSH_PARSER=y
 CONFIG_FASTBOOT=y
 CONFIG_USB_FUNCTION_FASTBOOT=y