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/rsk7203_defconfig b/configs/rsk7203_defconfig
index f2e9deb..f92536b 100644
--- a/configs/rsk7203_defconfig
+++ b/configs/rsk7203_defconfig
@@ -1,5 +1,6 @@
 CONFIG_SH=y
 CONFIG_TARGET_RSK7203=y
+CONFIG_VERSION_VARIABLE=y
 # CONFIG_AUTOBOOT is not set
 # CONFIG_CMD_BDI is not set
 # CONFIG_CMD_CONSOLE is not set