core/pkg-infra: allow per site-method sub-version strings

core/pkg-infra: allow per site-method sub-version strings

When we want to change the format of an archive we generate (e.g. those
we generate from git trees), the hashes of those archives will change.

To avoid any issue (e.g. an older Buildroot using newer archives, or the
other way around) that would conclude that the hashes do not match, we
want to change the filenames of the generated archives whenever we
change their format.

Introduce a new internal variable, specific to each site method, that we
can set to include a "format version" for the archives generated from
that site method.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Vincent Fazio <vfazio@xes-inc.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Reviewed-by: Vincent Fazio <vfazio@xes-inc.com>