distutils-common-base: add to, don't set, FILES_${PN}
If we set FILES_${PN} and a recipe inherits other classes that
modify FILES_${PN} *before* distutils-common-base is included, any
changes to FILES_${PN} made by those classes are lost.
Instead, append the additional directories we want to include in
FILES_${PN}
(From OE-Core rev:
f6478e8c73f9cfb79d1f7680b7bf3ff957eb51cb)
Signed-off-by: Joshua Lock <joshua.lock@collabora.co.uk>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
