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/bg0900_defconfig b/configs/bg0900_defconfig
index b3a13c6..ae54562 100644
--- a/configs/bg0900_defconfig
+++ b/configs/bg0900_defconfig
@@ -2,6 +2,7 @@
 CONFIG_TARGET_BG0900=y
 CONFIG_SPL=y
 CONFIG_BOOTDELAY=3
+CONFIG_VERSION_VARIABLE=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMLS is not set