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/armadillo-800eva_defconfig b/configs/armadillo-800eva_defconfig
index 3bd4680..7d0a592 100644
--- a/configs/armadillo-800eva_defconfig
+++ b/configs/armadillo-800eva_defconfig
@@ -2,6 +2,7 @@
 CONFIG_ARCH_RMOBILE=y
 CONFIG_TARGET_ARMADILLO_800EVA=y
 CONFIG_BOOTDELAY=3
+CONFIG_VERSION_VARIABLE=y
 # CONFIG_CMD_BDI is not set
 # CONFIG_CMD_CONSOLE is not set
 # CONFIG_CMD_BOOTD is not set