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/rainier_defconfig b/configs/rainier_defconfig
index 093207c..c0e647e 100644
--- a/configs/rainier_defconfig
+++ b/configs/rainier_defconfig
@@ -4,6 +4,7 @@
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_SYS_EXTRA_OPTIONS="RAINIER"
 CONFIG_BOOTDELAY=5
+CONFIG_VERSION_VARIABLE=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_ASKENV=y
 CONFIG_CMD_GREPENV=y