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/am335x_shc_defconfig b/configs/am335x_shc_defconfig
index c83311f..be9a99e 100644
--- a/configs/am335x_shc_defconfig
+++ b/configs/am335x_shc_defconfig
@@ -5,6 +5,7 @@
 CONFIG_SPL=y
 CONFIG_SPL_STACK_R=y
 CONFIG_FIT=y
+CONFIG_VERSION_VARIABLE=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="U-Boot# "
 CONFIG_AUTOBOOT_KEYED=y