imgtool: rollback to 1.7.0 release

Since there won't be anymore alpha releases, rollback to latest official
(from master). The next released versions now can be either 1.7.X from
v1.7-branch or X.Y.Zrc1 for the release candidate of next stable
version.

Signed-off-by: Fabio Utzig <utzig@apache.org>