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/m28evk_defconfig b/configs/m28evk_defconfig
index 4f7b648..bace4f6 100644
--- a/configs/m28evk_defconfig
+++ b/configs/m28evk_defconfig
@@ -3,6 +3,7 @@
 CONFIG_SPL=y
 CONFIG_FIT=y
 CONFIG_BOOTDELAY=3
+CONFIG_VERSION_VARIABLE=y
 CONFIG_HUSH_PARSER=y
 # CONFIG_CMD_IMLS is not set
 CONFIG_CMD_ASKENV=y