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/xtfpga_defconfig b/configs/xtfpga_defconfig
index 9a78bd5..129a62e 100644
--- a/configs/xtfpga_defconfig
+++ b/configs/xtfpga_defconfig
@@ -2,6 +2,7 @@
 CONFIG_SYS_CPU="dc233c"
 CONFIG_XTFPGA_KC705=y
 CONFIG_BOOTDELAY=10
+CONFIG_VERSION_VARIABLE=y
 CONFIG_SYS_PROMPT="U-Boot> "
 CONFIG_AUTOBOOT_KEYED=y
 CONFIG_AUTOBOOT_PROMPT="Autobooting in %d seconds, press <SPACE> to stop\n"