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/pepper_defconfig b/configs/pepper_defconfig
index 4a46f24..8a38b89 100644
--- a/configs/pepper_defconfig
+++ b/configs/pepper_defconfig
@@ -1,6 +1,7 @@
 CONFIG_ARM=y
 CONFIG_TARGET_PEPPER=y
 CONFIG_SPL=y
+CONFIG_VERSION_VARIABLE=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="pepper# "
 CONFIG_CMD_BOOTZ=y