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/porter_defconfig b/configs/porter_defconfig
index fe3f9d1..433b6ad 100644
--- a/configs/porter_defconfig
+++ b/configs/porter_defconfig
@@ -6,6 +6,7 @@
 CONFIG_BOOTSTAGE_STASH_ADDR=0x0
 CONFIG_BOOTSTAGE_STASH_SIZE=0x4096
 CONFIG_BOOTDELAY=3
+CONFIG_VERSION_VARIABLE=y
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMI is not set
 # CONFIG_CMD_IMLS is not set