cve-check: add json format

Add an option to output the CVE check in a JSON-based format.
This format is easier to parse in software than the original
text-based one and allows post-processing by other tools.

Output formats are now handed by CVE_CHECK_FORMAT_TEXT and
CVE_CHECK_FORMAT_JSON. Both of them are enabled by default.

The JSON output format gets generated in a similar way to the
text format with the exception of the manifest: appending to
JSON arrays requires parsing the file. Because of that we
first write JSON fragments and then assemble them in one pass
at the end.

Signed-off-by: Marta Rybczynska <marta.rybczynska@huawei.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>