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/tplink_wdr4300_defconfig b/configs/tplink_wdr4300_defconfig
index b1b069d..76a615c 100644
--- a/configs/tplink_wdr4300_defconfig
+++ b/configs/tplink_wdr4300_defconfig
@@ -4,6 +4,7 @@
 CONFIG_BOARD_TPLINK_WDR4300=y
 CONFIG_DEFAULT_DEVICE_TREE="tplink_wdr4300"
 CONFIG_BOOTDELAY=3
+CONFIG_VERSION_VARIABLE=y
 CONFIG_HUSH_PARSER=y
 # CONFIG_CMD_ELF is not set
 # CONFIG_CMD_IMLS is not set