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/mx28evk_nand_defconfig b/configs/mx28evk_nand_defconfig
index fc9fe9e..26c9762 100644
--- a/configs/mx28evk_nand_defconfig
+++ b/configs/mx28evk_nand_defconfig
@@ -3,6 +3,7 @@
 CONFIG_SPL=y
 CONFIG_SYS_EXTRA_OPTIONS="ENV_IS_IN_NAND"
 CONFIG_BOOTDELAY=1
+CONFIG_VERSION_VARIABLE=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMLS is not set