abuild: don't create junit tests with empty testclass field

Variable expansion made abuild create board..foo/bar, which are annoying
on jenkins' web UI because it doesn't cope properly with the empty
namespace between the dots. make it create board.foo/bar or
board.$class.foo/bar.

Change-Id: Ifa79cbfd1f263e11a458b3cc320baeed6a3fbc98
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Reviewed-on: http://review.coreboot.org/11640
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins)

util/abuild/abuild

@@ -479,7 +479,8 @@ chromeos=false
 clean_work=false
 customizing=""
 configoptions=""
-testclass=
+# testclass needs to be undefined if not used for variable expansion to work
+unset testclass
 while true ; do
 	case "$1" in
 		-J|--junit)     shift; mode=junit; rm -f $XMLFILE ;;