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/motionpro_defconfig b/configs/motionpro_defconfig
index eaaa5ed..add7c06 100644
--- a/configs/motionpro_defconfig
+++ b/configs/motionpro_defconfig
@@ -2,6 +2,7 @@
 CONFIG_MPC5xxx=y
 CONFIG_TARGET_MOTIONPRO=y
 CONFIG_OF_BOARD_SETUP=y
+CONFIG_VERSION_VARIABLE=y
 CONFIG_HUSH_PARSER=y
 CONFIG_AUTOBOOT_KEYED=y
 CONFIG_AUTOBOOT_PROMPT="Autobooting in %d seconds, press \"<Esc><Esc>\" to stop\n"