UBUNTU: snapcraft.yaml: various improvements

UBUNTU: snapcraft.yaml: various improvements

BugLink: http://bugs.launchpad.net/bugs/1700576
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 and linux-firmware-snapdragon packages in
   the final snap
2) rename it to snapdragon-kernel to match the snap we have in the store
3) dynamic versioning
4) autogenerated config from debian.$DEBIAN/config/*

Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>