MIPS: add MIPS Release 6 build coverage for Boston boards

Now that Travis CI is building with gcc-7.3.0, we can add
build coverage for all combinations of MIPS Release 6
instruction sets (MIPS32, MIPS64, Big Endian, Little Endian).

Add mew default configs for Boston board for all MIPS Release 6
variants.

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>