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_igep0033_defconfig b/configs/am335x_igep0033_defconfig
index d6022a3..efa3a9c 100644
--- a/configs/am335x_igep0033_defconfig
+++ b/configs/am335x_igep0033_defconfig
@@ -3,6 +3,7 @@
 CONFIG_SPL_STACK_R_ADDR=0x82000000
 CONFIG_SPL=y
 CONFIG_SPL_STACK_R=y
+CONFIG_VERSION_VARIABLE=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMLS is not set