include: unbreak conffiles, postinst & prerm exports

Changeset r43017 reworked the ipkg control metadata generation but broke
the export of conffiles, postinst and prerm defines.

Change the code back to rely on shvar and shexport, this is required to
properly output multiline contents.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

SVN-Revision: 43041