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/bcm28155_w1d_defconfig b/configs/bcm28155_w1d_defconfig
index b849074..8a5111f 100644
--- a/configs/bcm28155_w1d_defconfig
+++ b/configs/bcm28155_w1d_defconfig
@@ -2,6 +2,7 @@
 CONFIG_ENABLE_ARM_SOC_BOOT0_HOOK=y
 CONFIG_TARGET_BCM28155_AP=y
 CONFIG_SYS_EXTRA_OPTIONS="BCM_SF2_ETH,BCM_SF2_ETH_GMAC"
+CONFIG_VERSION_VARIABLE=y
 CONFIG_HUSH_PARSER=y
 # CONFIG_AUTOBOOT is not set
 CONFIG_CMD_BOOTZ=y