documenting the tarballs used for a build

[Matthias Klose]

Currently all tarballs downloaded are identified by a changeset ID, but there is
no documentation which changesets we do choose for a build. Could we document
this in some place, which version/tag is used for which tarball? Maybe just add
an extra field in hotspot.map? Or maybe rename hotspot.map to sources.map and
move the changeset id's and md5sums for the other tarballs into this file?

  Matthias