imagebuilder: Remove json_info_files/ before build

imagebuilder: Remove json_info_files/ before build

The folder `json_info_files` contains multiple JSON files which describe
created firmware images. The folder is not removed between builds as the
ImageBuilder does not use `image.mk`.

Not removing the JSON files result in a merged `profiles.json` file
containing entries for outdated or non-existing images.

This commit adds the `json_info_files/` cleanup step to the ImageBuilder
Makefile.

Signed-off-by: Paul Spooren <mail@aparcar.org>
(cherry picked from commit 941ec28b355ea690b5682d2bf8175aa513379997)