build,json: fix duplicates in default_packages

Calling without the DUMP=1 argument causes the target specific Makefile
to be "included" again which adds the target specific packages twice,
once on the actual run and once included from `include/target.mk`.

This led to duplicate package entries, causing confusion in downstream
projects using the generated JSON files.

While at it, apply `black` style to Python script.

Signed-off-by: Paul Spooren <mail@aparcar.org>
(cherry picked from commit 7f4c2b1a4f9216218dced64794318f2197565c85)

scripts/json_overview_image_info.py

@@ -38,9 +38,10 @@ if output:
             "make",
             "--no-print-directory",
             "-C",
-            "target/linux/{}".format(output['target'].split('/')[0]),
+            "target/linux/{}".format(output["target"].split("/")[0]),
             "val.DEFAULT_PACKAGES",
             "val.ARCH_PACKAGES",
+            "DUMP=1",
         ],
         stdout=PIPE,
         stderr=PIPE,