Need variable for arch triplet in yaml

[Michi Henning]

For the thumbnailer, we need to copy additional libraries into our prime. The problem is that there doesn't appear to be a generic way to refer to the lib directory from inside the yaml. In turn, this leads to ugly hackery such as this:

plugin=$(echo /usr/lib/*/gstreamer-1.0/libgstlibav.so)
mkdir -p $SNAPCRAFT_PART_INSTALL/$(dirname $plugin)
cp $plugin $SNAPCRAFT_PART_INSTALL/$plugin

It would be a lot clearer and more direct to to have an ARCH_TRIPLET variable, so I could say what I mean.

[Leo Arias (elopio)]

This makes sense, I think I'd find the triplet useful too.
However, note here: https://github.com/snapcore/snapcraft/blob/master/snapcraft/_options.py#L29
that there are many values, and maybe some snaps will want the deb arch instead of the triplet.

We'll have to discuss if we want to expose all those as variables.

[Michi Henning (michihenning)]

Having the deb arch in addition to the triplet would be useful, too.