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/am57xx_evm_nodt_defconfig b/configs/am57xx_evm_nodt_defconfig
index fb0c6bc..0066691 100644
--- a/configs/am57xx_evm_nodt_defconfig
+++ b/configs/am57xx_evm_nodt_defconfig
@@ -2,6 +2,7 @@
 CONFIG_OMAP54XX=y
 CONFIG_TARGET_AM57XX_EVM=y
 CONFIG_SPL=y
+CONFIG_VERSION_VARIABLE=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMLS is not set