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/legoev3_defconfig b/configs/legoev3_defconfig
index 8161f73..5ab3a61 100644
--- a/configs/legoev3_defconfig
+++ b/configs/legoev3_defconfig
@@ -2,6 +2,7 @@
 CONFIG_ARCH_DAVINCI=y
 CONFIG_TARGET_LEGOEV3=y
 CONFIG_BOOTDELAY=0
+CONFIG_VERSION_VARIABLE=y
 CONFIG_HUSH_PARSER=y
 CONFIG_AUTOBOOT_KEYED=y
 CONFIG_AUTOBOOT_PROMPT="Autoboot in %d seconds - press 'l' to stop...\n"