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_sl50_defconfig b/configs/am335x_sl50_defconfig
index 01c1eeb..dae8f1b 100644
--- a/configs/am335x_sl50_defconfig
+++ b/configs/am335x_sl50_defconfig
@@ -5,6 +5,7 @@
 CONFIG_SPL_STACK_R=y
 CONFIG_FIT=y
 CONFIG_SYS_EXTRA_OPTIONS="EMMC_BOOT"
+CONFIG_VERSION_VARIABLE=y
 CONFIG_HUSH_PARSER=y
 # CONFIG_AUTOBOOT is not set
 CONFIG_CMD_BOOTZ=y