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/ms7720se_defconfig b/configs/ms7720se_defconfig
index 537ea7c..a02760a 100644
--- a/configs/ms7720se_defconfig
+++ b/configs/ms7720se_defconfig
@@ -1,5 +1,6 @@
 CONFIG_SH=y
 CONFIG_TARGET_MS7720SE=y
+CONFIG_VERSION_VARIABLE=y
 # CONFIG_AUTOBOOT is not set
 # CONFIG_CMD_BDI is not set
 # CONFIG_CMD_CONSOLE is not set