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/ap_sh4a_4a_defconfig b/configs/ap_sh4a_4a_defconfig
index 73e6f04..ee14e96 100644
--- a/configs/ap_sh4a_4a_defconfig
+++ b/configs/ap_sh4a_4a_defconfig
@@ -1,6 +1,7 @@
 CONFIG_SH=y
 CONFIG_TARGET_AP_SH4A_4A=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