Similarly to what native.bbclass does, clear TUNE_FEATURES since these
aren't appropriate to the nativesdk build. This saves us having to change
down signature issues due to data in this variable.
(From OE-Core rev: 2e5079e434504a3a22b609f7ddbb7f4c533c4cdb)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>