don't use build.list to detect multi-tree projects

instead, check for the presence of target.cfg files
not in config/project/ but config/project/tree/

the way this check is done, it merely returns 1 if
config/project/*/target.cfg is detected, and returns
0 in all other cases, even if config/project/target.cfg
exists

that way, if the maintainer accidentally adds a
target.cfg in the main directory, the given multi-tree
project will not break

Signed-off-by: Leah Rowe <leah@libreboot.org>

script/trees

@@ -39,7 +39,7 @@ main()
 	remkdir "${tmpgit%/*}"
 
 	_cmd="build_projects"
-	[ -f "config/$project/build.list" ] && _cmd="build_targets"
+	singletree "$project" || _cmd="build_targets"
 	$_cmd $@
 }