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/PMC440_defconfig b/configs/PMC440_defconfig
index 552fdd2..af38882 100644
--- a/configs/PMC440_defconfig
+++ b/configs/PMC440_defconfig
@@ -3,6 +3,7 @@
 CONFIG_TARGET_PMC440=y
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_BOOTDELAY=3
+CONFIG_VERSION_VARIABLE=y
 CONFIG_AUTOBOOT_KEYED=y
 CONFIG_AUTOBOOT_PROMPT="Press SPACE to abort autoboot in %d seconds\n"
 CONFIG_AUTOBOOT_STOP_STR=" "