cve-check: add include/exclude layers

There are times when exluding or including a layer
may be desired. This provide the framwork for that via
two variables. The default is all layers in bblayers.

CVE_CHECK_LAYER_INCLUDELIST
CVE_CHECK_LAYER_EXCLUDELIST

Signed-off-by: Armin Kuster <akuster808@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>