Add description to MAINBOARD_VENDOR string so it can be overridden

A quirk of the Kconfig used in coreboot is that config options
cannot be overriden by local config changes unless they have
a description string.

1) Add CONFIG_MAINBOARD_VENDOR="Custom" to local config
2) Build and flash coreboot
3) cat /sys/class/dmi/id/sys_vendor and look for "Custom"

Change-Id: I1b5f2124cd4a22c056c025143ae5bcaafa6b03f0
Signed-off-by: Duncan Laurie <dlaurie@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/59088
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/4248
Tested-by: build bot (Jenkins)
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>

src/mainboard/google/Kconfig

@@ -58,7 +58,7 @@ source "src/mainboard/google/snow/Kconfig"
 source "src/mainboard/google/stout/Kconfig"
 
 config MAINBOARD_VENDOR
-	string
+	string "Mainboard Vendor"
 	default "Google"
 
 endif # VENDOR_GOOGLE