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/cm_t54_defconfig b/configs/cm_t54_defconfig
index 8df06e3..18bb04e 100644
--- a/configs/cm_t54_defconfig
+++ b/configs/cm_t54_defconfig
@@ -4,6 +4,7 @@
 CONFIG_SPL=y
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_BOOTDELAY=3
+CONFIG_VERSION_VARIABLE=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="CM-T54 # "
 CONFIG_CMD_BOOTZ=y