build: Have `hydra.scm' return one job per package.

* hydra.scm (hydra-jobs): Return one job for each non-bootstrap package.

build: Fix `hydra.scm' recipe.

* hydra.scm: Redirect the output port to the error port.
  (package-job): Return a name/thunk pair.
  (hydra-jobs)[system]: Use either the `system' key (a symbol) in
  ARGUMENTS, or (%current-system)'.

build: Add `hydra.scm'.

* hydra.scm: New file.
* Makefile.am (EXTRA_DIST): Add it.