conf/mips: mips16e: prepend override to MACHINEOVERRIDES

Add the tune specific override to MACHINEOVERRIDES and not OVERRIDES as
is done for all other tune include files.

Also prepend it instead of appending so that it's among the leftmost
overrides in MACHINEOVERRIDES and has a lower precedence compared to
other MACHINEOVERRIDES added later (which usually are added via a =.
(prepend)).

Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>