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/bcm958305k_defconfig b/configs/bcm958305k_defconfig
index df76dac..ccdbd31 100644
--- a/configs/bcm958305k_defconfig
+++ b/configs/bcm958305k_defconfig
@@ -1,6 +1,7 @@
 CONFIG_ARM=y
 CONFIG_TARGET_BCMCYGNUS=y
 CONFIG_SYS_EXTRA_OPTIONS="SYS_SDRAM_SIZE=0x40000000"
+CONFIG_VERSION_VARIABLE=y
 CONFIG_HUSH_PARSER=y
 # CONFIG_AUTOBOOT is not set
 CONFIG_CMD_BOOTZ=y