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/sh7763rdp_defconfig b/configs/sh7763rdp_defconfig
index 41d2b55..5957a56 100644
--- a/configs/sh7763rdp_defconfig
+++ b/configs/sh7763rdp_defconfig
@@ -1,6 +1,7 @@
 CONFIG_SH=y
 CONFIG_TARGET_SH7763RDP=y
 CONFIG_BOOTDELAY=-1
+CONFIG_VERSION_VARIABLE=y
 # CONFIG_CMD_BDI is not set
 # CONFIG_CMD_CONSOLE is not set
 # CONFIG_CMD_BOOTD is not set