build: refactor JSON info files to `profiles.json`

JSON info files contain machine readable information of built profiles
and resulting images. These files were added in commit 881ed09ee6e2
("build: create JSON files containing image info").

They are useful for firmware wizards and script checking for
reproducibility.

Currently all JSON files are stored next to the built images, resulting
in up to 168 individual files for the ath79/generic target.

This patch refactors the JSON creation to store individual per image
(not per profile) files in $(BUILD_DIR)/json_info_files and create an
single overview file called `profiles.json` in the target directory.

Storing per image files and not per profile solves the problem of
parallel file writes. If a profiles sysupgrade and factory image are
finished at the same time both processes would write to the same JSON
file, resulting in randomly broken outputs.

Some target like x86/64 do not use the image code yet, resulting in
missing JSON files. If no JSON info files were created, no
`profiles.json` files is created as it would be empty anyway.

As before, this creation is enabled by default only if `BUILDBOT` is set.

Tested via buildroot & ImageBuilder on ath79/generic, imx6 and x86/64.

Signed-off-by: Paul Spooren <mail@aparcar.org>
[json_info_files dir handling in Make, if case refactoring]
Signed-off-by: Petr Štetiar <ynezz@true.cz>

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