tune-core2: Update qemu cpu to supported model

tune-core2: Update qemu cpu to supported model

Fixes [YOCTO #12388]

QEMUs documentation does recommend to not use n270 and core2duo as
an argument to -cpu anymore. See also the QEMU documentation for this at
[0].
Update therefore the QEMU cpu option for the core2duo tune to Nehalam.
Tested it locally with QEMU and KVM.

[0]: https://qemu-project.gitlab.io/qemu/system/qemu-cpu-models.html#other-non-recommended-x86-cpus

Signed-off-by: Simone Weiß <simone.p.weiss@posteo.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>