This bug has been around for a long time (2011) but fix it to do
what was intended originally. The postrm is changed by classes but
those should append to existing entries.
(From OE-Core rev: 7762b07bf3b0c77b91ad91963ab39d3c73770ebf)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>