machine_kernel_pr.bbclass: provides MACHINE_KERNEL_PR functionality

Diffing with the OE-Core's kernel.bbclass only difference is setting of
machine kernel pr. So lets convert it to a separate class which provides
the added functionality and let everyone use kernel.bbclass from kernel

It needs the dependent layers to inherit this class in addition to
kernel.bbclass.

After dependent layers are updated we can remove meta-oe copy of
kernel.bbclass, it leaves up with one less conflict with OE-Core.

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>