Clarify MIPS ABIs support

Practically speaking, MIPS has three useful ABIs:

 * o32 is for 32-bits CPUs, or 64-bit CPUs running only a 32-bit subset
   of the instruction set.
 * n32 is for 64-bits CPUs only. It has 32-bits pointers and long
   integers.
 * n64 is for 64-bits CPUs only. It has 64-bits pointers and long
   integers.

See http://www.linux-mips.org/wiki/MIPS_ABI_History and
http://www.linux-mips.org/wiki/WhatsWrongWithO32N32N64 for more
details.

So, this commit reworks the Buildroot MIPS support by:

 * Add separate mips64/mips64el top-level architectures.

 * Renaming the n32 ABI option to BR2_MIPS_NABI32, for consistency
   with BR2_MIPS_OABI32.

 * Renaming the n64 ABI option to BR2_MIPS_NABI64, for consistency
   with BR2_MIPS_OABI32.

 * Make the n32 and n64 ABI selections select the BR2_ARCH_IS_64,
   since those ABIs are valid on 64-bits CPUs only.

 * Removing the o64 ABI, which is practicaly never used.

 * Removing the "none" ABI, which really doesn't make sense.

 * Introduce the mips64 and mips64el architecture names when a 64-bits
   MIPS ABI is choosen. This will fix build issue like
   http://autobuild.buildroot.org/results/9b8c5ea86c953a89e85e7b67e9221de41773f652/build-end.log
   where gmp was confused by the fact of having a 32 bits architecture
   (detected by the mips- architecture part of the tuple) but 64 bits
   integer size when compiling.

 * Adjust the uclibc.mk logic to support the new mips64/mips64el
   architecture names, and take into account the renaming of the ABI
   options.

This has been build tested by generating Buildroot toolchains and
compiling a few packages for MIPS o32, MIPS n32 and MIPS n64.

This work is originally based on prior work done by Gustavo Zacarias.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

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