cve-check: Introduce CVE_CHECK_MANIFEST_JSON_SUFFIX

The variable contains the suffix of the CVE JSON manifest file.
By default, this variable is set to 'json', so the current behavior
is not changed, but enables developers to use some other suffix,
e.g., cve.json (similar to spdx.json).

Signed-off-by: Aleksandar Nikolic <an010@live.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>