build,json: fix duplicates in default_packages

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)