Don't allow both ENDIANNESSes to be set

See discussion on https://github.com/micropython/micropython/commit/2da81fa80c4cd965f05ad237d81d9764322fde20

Explicitly set `MP_ENDIANNESS_LITTLE` because that's the #define that is used in code elsewhere.
Merge pull request #479 from lurch/patch-1

Don't allow both ENDIANNESSes to be set