MIPS: Refactor legacy defconfigs

MIPS: Refactor legacy defconfigs

Define legacy defconfigs which have been converted to the generic
platform more programatically, so that they can be listed in the
Makefile help text and as a separate Makefile target without
duplication.

Signed-off-by: James Hogan <jhogan@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Paul Burton <paul.burton@mips.com>
Cc: Matt Redfearn <matt.redfearn@mips.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kbuild@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/18596/