vex: fix rootfs manifest

vex: fix rootfs manifest

Rootfs VEX file is created by gathering files from CVE_CHECK_DIR
(deploy directory), however recipes generate the files only in
CVE_CHECK_DIR (log directory).
This make the rootfs VEX be always empty without any message.

The code is copied from cve_check class, which writes to both, so let
keep them aligned and make also vex write both files.

Also add a warning for case that a cve file would be still missing.

Signed-off-by: Peter Marko <peter.marko@siemens.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>