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/am3517_evm_defconfig b/configs/am3517_evm_defconfig
index 292b776..d5a0e30 100644
--- a/configs/am3517_evm_defconfig
+++ b/configs/am3517_evm_defconfig
@@ -5,6 +5,7 @@
 CONFIG_FIT=y
 CONFIG_SYS_EXTRA_OPTIONS="NAND"
 CONFIG_BOOTDELAY=10
+CONFIG_VERSION_VARIABLE=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="AM3517_EVM # "
 CONFIG_CMD_BOOTZ=y