insane.bbclass: Don't let warnings make previous errors non-fatal

insane.bbclass: Don't let warnings make previous errors non-fatal

package_qa_handle_error() returns True on non-fatal issues and False on
fatal issues. But the current usage has been to do

  sane = package_qa_handle_error(...)

which would always reset sanity status to be that of the last issue
identified. This change the assignments to use the &= operator instead:

  sane &= package_qa_handle_error(...)

As far as I can tell, this is not a real problem in practice, because
warnings of different levels (WARN_QA, ERROR_QA) does not seem to have
been mixed in a way that triggered this issue.

Signed-off-by: Olof Johansson <olofjn@axis.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>