This also updates comments and removes the comment about a very old
`bootstrap-profile' hack.
* gnu/system/examples/devel-hurd.tmpl (hurd-packages): New variable
(%hurd-devel-os): Use it.
* gnu/system/examples/devel-hurd64.tmpl: New file.
Change-Id: I54b6b0843f0dd635d89cca483ae43d23b20d21e8