UBUNTU: snapcraft.yaml: various improvements

UBUNTU: snapcraft.yaml: various improvements

BugLink: http://bugs.launchpad.net/bugs/1700480
This patch improves over the original snapcraft.yaml we ship in Xenial,
and makes the generated kernel snap nearly identical to the one we ship
in the store.
Among the different improvements, we have:

1) include the linux-firmware package in the final snap
2) rename it to pc-kernel to match the snap we have in the store
3) dynamic versioning
4) autogenerated config from debian.$DEBIAN/config/*
5) generate from the same snap versions for i386, amd64, armhf, arm64, ppc64el
and s390x

Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>
Acked-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Shrirang Bagul <shrirang.bagul@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>