feature-freeze for release v4.8

Merge pull request contiki-ng#1608 from contiki-ng/release-4.7

Release 4.7

created dev-tag for v4.7 release

Release v4.6

Start of release branch for v4.6

Release v4.5

Start of release branch for v4.5

Release v4.4

Start of release branch for v4.4

Release v4.3