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/portl2_defconfig b/configs/portl2_defconfig
index 87fd221..94f9349 100644
--- a/configs/portl2_defconfig
+++ b/configs/portl2_defconfig
@@ -2,6 +2,7 @@
 CONFIG_KIRKWOOD=y
 CONFIG_TARGET_KM_KIRKWOOD=y
 CONFIG_SYS_EXTRA_OPTIONS="KM_PORTL2"
+CONFIG_VERSION_VARIABLE=y
 CONFIG_HUSH_PARSER=y
 CONFIG_AUTOBOOT_KEYED=y
 CONFIG_AUTOBOOT_PROMPT="Hit <SPACE> key to stop autoboot in %2ds\n"