libopkg: factor out checksum and size verification

libopkg: factor out checksum and size verification

This is a sizeable chunk of code that be can pretty well isolated in its
own function.

This refactoring will be necessary for an upcoming feature in which
opkg_download_pkg() will need to verify the checksum of packages in the
cache.  This is the reason why the new function is located in
`opkg_download.c`.

Signed-off-by: Baptiste Jonglez <git@bitsofnetworks.org>
(cherry picked from commit 293b1cef8b5a6de64d3fec87c01729b64006713a)