trees: skip single-tree build if a build exists

this replicates the same behaviour as multi-tree builds,
checking for files inside the relevant elf/ directory

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

script/trees

@@ -59,6 +59,7 @@ build_projects()
 	[ -f "$listfile" ] || listfile="" # optional on single-tree
 
 	dest_dir="$elfdir"
+	elfcheck || return 0
 
 	cdir="src/${project}"
 	[ -d "$cdir" ] || x_ ./update trees -f "$project"
@@ -173,11 +174,17 @@ check_config()
 	[ -f "$config" ] || $err "check_config $project/$target: no config"
 
 	dest_dir="$elfdir/$target/$config_name"
+	elfcheck || return 1 # skip build if a previous one exists
+
+	x_ mkdir -p "$dest_dir"
+}
+
+elfcheck()
+{
 	# TODO: very hacky check. do it properly (based on build.list)
 	for elftest in "$dest_dir"/*; do
 		e "$elftest" f && return 1
 	done
-	x_ mkdir -p "$dest_dir"
 }
 
 handle_makefile()