nativesdk: don't unset MACHINE_FEATURES, let machine-sdk/ set it

nativesdk: don't unset MACHINE_FEATURES, let machine-sdk/ set it

There is actually a use for nativesdk MACHINE_FEATURES; for example
qemu-usermode being supported, as this is needed to build profile-guided
optimised code.

We shouldn't use the target MACHINE_FEATURES for this because the target
and the SDK can be entirely different, so instead set the
MACHINE_FEATURES in nativesdk.bbclass to SDK_MACHINE_FEATURES (which
defaults to "") and let the conf/machine-sdk/*.conf files set that as
appropriate.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Link: https://lore.kernel.org/r/20231221170159.1995650-2-ross.burton@arm.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>