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/makalu_defconfig b/configs/makalu_defconfig
index 4967e10..bb9ee2f 100644
--- a/configs/makalu_defconfig
+++ b/configs/makalu_defconfig
@@ -3,6 +3,7 @@
 CONFIG_TARGET_MAKALU=y
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_BOOTDELAY=5
+CONFIG_VERSION_VARIABLE=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_ASKENV=y
 CONFIG_CMD_GREPENV=y