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/ids8313_defconfig b/configs/ids8313_defconfig
index b0487ef..02ca830 100644
--- a/configs/ids8313_defconfig
+++ b/configs/ids8313_defconfig
@@ -7,6 +7,7 @@
 CONFIG_OF_STDOUT_VIA_ALIAS=y
 CONFIG_SYS_EXTRA_OPTIONS="SYS_TEXT_BASE=0xFFF00000"
 CONFIG_BOOTDELAY=1
+CONFIG_VERSION_VARIABLE=y
 CONFIG_HUSH_PARSER=y
 CONFIG_AUTOBOOT_KEYED=y
 CONFIG_AUTOBOOT_PROMPT="Enter password - autoboot in %d seconds...\n"