trees: don't do elfcheck if build.list missing

otherwise, some checks are done too soon, and nothing
gets built.

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

script/trees

@@ -60,7 +60,7 @@ build_projects()
 	[ -f "$listfile" ] || listfile="" # optional on single-tree
 
 	dest_dir="$elfdir"
-	elfcheck || return 0
+	[ ! -f "$listfile" ] || elfcheck || return 0
 
 	cdir="src/${project}"
 	[ -d "$cdir" ] || x_ ./update trees -f "$project"