boot: Update copyrights and licenses

To make contributions easier, place explicit copyrights by the major
contributors, along with an SPDX license identifier.  Files that came
from the mynewt project, which was an Apache project will retain the
Apache project license text, although this does not apply to new
contributions, which are being made by individual contributors.

Hopefully, this will keep everyone happy, but allow contributors that
need to add an explicit copyright to have a place they can add that.

Fixes #501

Signed-off-by: David Brown <david.brown@linaro.org>