package/canfestival: switch to using 'hg' to download

Currently, the canfestival package downloads a archive generated by the
remote Mercurial server.

This has the unfortuante side-effect of downloading an archive name just
after the revision string, without the package name in it. So, users do
have this archive in their BR2_DL_DIR: 7740ac6fdedc.tar.bz2

Switch to doing an hg clone to download canfestival, so we end up with a
properly named archive. Also expand the hash to the full-lenght hash
rather than the shorten one used when asking the remote server to
generate the archive.

[Thomas: fix typo in commit log.]

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Samuel Martin <s.martin49@gmail.com>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>