insane.bbclass: make package_qa_clean_path return a relative path

insane.bbclass: make package_qa_clean_path return a relative path

Make package_qa_clean_path() return something like "work/path/to/file"
rather than "/work/path/to/file", the relative path is a little clear.

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>