cve_check: skip remote patches that haven't been fetched when searching for CVE tags

cve_check: skip remote patches that haven't been fetched when searching for CVE tags

If a remote patch is compressed we need to have run the unpack task for
the file to exist locally.  Currently cve_check only depends on fetch so
instead of erroring out, emit a warning that this file won't be scanned
for CVE references.

Typically, remote compressed patches won't contain our custom tags, so
this is unlikely to be an issue.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit cefc8741438c91f74264da6b59dece2e31f9e5a5)
Signed-off-by: Steve Sakoman <steve@sakoman.com>