Disable imgtool publishing on Travis; update scripts to work on both
Travis and GH; add GH workflow for publishing.
Signed-off-by: Fabio Utzig <fabio.utzig@nordicsemi.no>
Add new CI vm that parses the imgtool version from __init__.py and
compares with the current published release. If the version in the repo
is newer it will be published automatically.
Signed-off-by: Fabio Utzig <utzig@apache.org>