cve-check: Fix report generation

cve-check: Fix report generation

The addition of summary output caused two issues: error when building
an image and the fact that JSON output was generated even when
CVE_CHECK_FORMAT_JSON.

When generating an image it caused an error like:
ERROR: core-image-minimal-1.0-r0 do_rootfs: Error executing a python function in exec_func_python() autogenerated:

  The stack trace of python calls that resulted in this exception/failure was:
  File: 'exec_func_python() autogenerated', lineno: 2, function: <module>
       0001:
   *** 0002:cve_check_write_rootfs_manifest(d)
       0003:
  File: '/home/alexk/poky/meta/classes/cve-check.bbclass', lineno: 213, function: cve_check_write_rootfs_manifest
       0209:
       0210:        link_path = os.path.join(deploy_dir, "%s.json" % link_name)
       0211:        manifest_path = d.getVar("CVE_CHECK_MANIFEST_JSON")
       0212:        bb.note("Generating JSON CVE manifest")
   *** 0213:        generate_json_report(json_summary_name, json_summary_link_name)
       0214:        bb.plain("Image CVE JSON report stored in: %s" % link_path)
       0215:}
       0216:
       0217:ROOTFS_POSTPROCESS_COMMAND:prepend = "${@'cve_check_write_rootfs_manifest; ' if d.getVar('CVE_CHECK_CREATE_MANIFEST') == '1' else ''}"
  Exception: NameError: name 'json_summary_name' is not defined

The fix is to pass the d variable to the pure python function generate_json_report
to get correct values of variables and add conditions for the JSON
output where needed.

In addition clarify the message presenting the summary JSON file,
which isn't related to an image.

Uses partial fixes from Alex Kiernan, Ernst Sjöstrand (ernstp),
and Davide Gardenal.

Fixes: f2987891d315 ("cve-check: add JSON format to summary output")
Signed-off-by: Marta Rybczynska <marta.rybczynska@huawei.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>