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/ti816x_evm_defconfig b/configs/ti816x_evm_defconfig
index c4240e9..7df29cc 100644
--- a/configs/ti816x_evm_defconfig
+++ b/configs/ti816x_evm_defconfig
@@ -2,6 +2,7 @@
 CONFIG_TARGET_TI816X_EVM=y
 CONFIG_SPL=y
 CONFIG_BOOTDELAY=3
+CONFIG_VERSION_VARIABLE=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="u-boot/ti816x# "
 # CONFIG_CMD_IMLS is not set