Provide a workaround for setting the correct endianness when doing CFI on
a mips big-endian board.

This is (hopefully! ish!) a temporary change until a slightly better way
can be found to express this without a config option.

Tested:

• BUFFALO WZR-HP-G300NH 1stGen (by submitter)

Submitted by: Mori Hiroki <yamori813@yahoo.co.jp>