insane.bbclass: Add a check for directories that are expected to be empty

The empty-dirs QA check verifies that all directories specified in
QA_EMPTY_DIRS are empty. It is possible to specify why a directory is
expected to be empty by defining QA_EMPTY_DIRS_RECOMMENDATION:<path>,
which will then be included in the error message if the directory is
not empty. If it is not specified for a directory, then "but it is
expected to be empty" will be used.

Signed-off-by: Peter Kjellerstedt <peter.kjellerstedt@axis.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>