pkg-download: verify the hashes from the download wrapper

pkg-download: verify the hashes from the download wrapper

Instead of repeating the check in our download rules, delegate the check
of the hashes to the download wrapper.

This needs three different changes:

  - add a new argument to the download wrapper, that is the full path to
    the hash file; if the hash file does not exist, that does not change
    the current behaviour, as the existence of the hash file is checked
    for in the check-hash script;

  - add a third argument to the check-hash script, to be the basename of
    the file to check; this is required because we no longer check the
    final file with the final filename, but an intermediate file with a
    temporary filename;

  - do the actual call to the check-hash script from within the download
    wrapper.

This further paves the way to doing pre-download checks of the hashes
for the locally cached files.

Note: this patch removes the check for hashes for already downloaded
files, since the wrapper script exits early. The behaviour to check
localy cached files will be restored and enhanced in the following
patch.

[Thomas: fix minor typo in comment.]

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Cc: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>