package.bbclass: return list of packages created in do_split_packages

* sometimes it's useful to do something with packages created
  by do_split_packages later in do_package_prepend, e.g. in:
  http://lists.linuxtogo.org/pipermail/openembedded-devel/2013-February/043824.html
  I have .bbclass which adds some postinst, postrm and RDEPENDS,
  but ttf-mplus is using do_split_packages to create those packages

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>