cve-check: restore CVE_CHECK_SHOW_WARNINGS functionality

cve-check: restore CVE_CHECK_SHOW_WARNINGS functionality

Commit 05ef4f2a7b225c8d230eaca8d333ffb921729d79 removed this
functionality by accident. It was implemented in text exporter, while it
should have been a global feature independent on exporter type to avoid
such accidental deletion.

Signed-off-by: Peter Marko <peter.marko@siemens.com>
Cc: Marta Rybczynska <marta.rybczynska@ygreky.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 2996b11596afca288a6b7f409a5287063d331f3b)
Signed-off-by: Steve Sakoman <steve@sakoman.com>