cve-check: introduce CVE_CHECK_RECIPE_FILE variable to allow changing of per-recipe check file

The addition of this variable also makes it possible to change the
output suffix of the check files, e.g. in local.conf:

CVE_CHECK_MANIFEST_append = ".txt"
CVE_CHECK_RECIPE_FILE_append = ".txt"

Signed-off-by: Chris Laplante <chris.laplante@agilent.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>