configure.py: Write last key in each section

The loop that writes the keys in each section of bootstrap.toml
accumulates all the commented lines before a given key and emits them
when it reaches the next key in the section.  This ends up dropping
lines accumulated for the last key

src/bootstrap/configure.py

@@ -752,6 +752,10 @@ def write_uncommented(target, f):
             is_comment = True
             continue
         is_comment = is_comment and line.startswith("#")
+    # Write the last accumulated block
+    if len(block) > 0 and not is_comment:
+        for ln in block:
+            f.write(ln + "\n")
     return f