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/da850_am18xxevm_defconfig b/configs/da850_am18xxevm_defconfig
index 69839df..ce49340 100644
--- a/configs/da850_am18xxevm_defconfig
+++ b/configs/da850_am18xxevm_defconfig
@@ -4,6 +4,7 @@
 CONFIG_SPL=y
 CONFIG_SYS_EXTRA_OPTIONS="DA850_AM18X_EVM,MAC_ADDR_IN_EEPROM,SYS_I2C_EEPROM_ADDR_LEN=2,SYS_I2C_EEPROM_ADDR=0x50"
 CONFIG_BOOTDELAY=3
+CONFIG_VERSION_VARIABLE=y
 CONFIG_HUSH_PARSER=y
 # CONFIG_CMD_IMLS is not set
 CONFIG_CMD_ASKENV=y