diff --git a/core/product_config.rbc b/core/product_config.rbc
index f26b428..d47cdc0 100644
--- a/core/product_config.rbc
+++ b/core/product_config.rbc
@@ -476,6 +476,12 @@
     return sorted(["%s/%s:%s/%s" % (
         from_dir, f, to_dir, f) for f in rblf_find_files(from_dir, pattern, only_files=1)])
 
+def _findstring(needle, haystack):
+    """Equivalent to GNU make's $(findstring)."""
+    if haystack.find(needle) < 0:
+        return ""
+    return needle
+
 def _filter_out(pattern, text):
     """Return all the words from `text' that do not match any word in `pattern'.
 
@@ -727,6 +733,7 @@
     filter = _filter,
     filter_out = _filter_out,
     find_and_copy = _find_and_copy,
+    findstring = _findstring,
     inherit = _inherit,
     indirect = _indirect,
     mk2rbc_error = _mk2rbc_error,