Expand the MIPS Makefile help text to list generic board names, generic
defconfigs, and legacy defconfigs which have been converted to generic
and are still usable.

Here's a snippet of the new "make ARCH=mips help" output:
  ...
  If you are targeting a system supported by generic kernels you may
  configure the kernel for a given architecture target like so:

  {micro32,32,64}{r1,r2,r6}{el,}_defconfig <BOARDS="list of boards">

  Where BOARDS is some subset of the following:
    boston
    ni169445
    ranchu
    sead-3
    xilfpga

  Specifically the following generic default configurations are
  supported:

  32r1_defconfig           - Build generic kernel for MIPS32 r1
  32r1el_defconfig         - Build generic kernel for MIPS32 r1 little endian
  32r2_defconfig           - Build generic kernel for MIPS32 r2
  32r2el_defconfig         - Build generic kernel for MIPS32 r2 little endian
  32r6_defconfig           - Build generic kernel for MIPS32 r6
  32r6el_defconfig         - Build generic kernel for MIPS32 r6 little endian
  64r1_defconfig           - Build generic kernel for MIPS64 r1
  64r1el_defconfig         - Build generic kernel for MIPS64 r1 little endian
  64r2_defconfig           - Build generic kernel for MIPS64 r2
  64r2el_defconfig         - Build generic kernel for MIPS64 r2 little endian
  64r6_defconfig           - Build generic kernel for MIPS64 r6
  64r6el_defconfig         - Build generic kernel for MIPS64 r6 little endian
  micro32r2_defconfig      - Build generic kernel for microMIPS32 r2
  micro32r2el_defconfig    - Build generic kernel for microMIPS32 r2 little endian

  The following legacy default configurations have been converted to
  generic and can still be used:

  sead3_defconfig          - Build 32r2el_defconfig BOARDS=sead-3
  sead3micro_defconfig     - Build micro32r2el_defconfig BOARDS=sead-3
  xilfpga_defconfig        - Build 32r2el_defconfig BOARDS=xilfpga
  ...
Signed-off-by: NJames 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/18598/