Merge branch 'core-updates'

build: Add "mips64el-linux-gnuabi64" as a cross build target.

* build-aux/hydra/gnu-system.scm (%packages-to-cross-build): Add
  %bootstrap-tarballs.
  (%cross-targets): Add "mips64el-linux-gnuabi64".

build: Build the bootstrap tarballs on Hydra.

* build-aux/hydra/gnu-system.scm (%packages-to-cross-build): Uncomment
  the bootstrap tarball packages.

Merge branch 'master' into core-updates

Conflicts:
	gnu/packages/make-bootstrap.scm

build: Move Hydra recipe to build-aux/hydra.

* hydra.scm: Rename to...
* build-aux/hydra/gnu-system.scm: ... this.
* Makefile.am (EXTRA_DIST): Adjust accordingly.