Update spec changelog for 0.8.1

I'm retroactively tagging a previous commit (e2a223e) as 0.8.1; this is
the changelog for that release.
Bump version to 0.9.0

Gotta bump the version to differentiate the git version from the
soon-to-be-released 0.8.1. And the new handling of config files for
upgrade.img is sufficiently different that I think this deserves 0.9.0.
improve "make archive" and add "make snapshot"

"make archive" makes an archive for the version specified in $(RELEASE_TAG).

"make snapshot" makes an archive for HEAD, using $(VERSION) and adding
some stuff to make it clear that this is a pre-release tarball.