build: envkernel: support packaging kernels that were built on the host

Add support for packaging a kernel that was compiled outside of envkernel. The envkernerl wrapper impacts speed and can make things like running dt_binding_check frustrating, by just bind mounting the source dir in the same way that envkernel does the rest of the packaging logic works just fine.

This is sort of a like pmbootstrap build --src except it only does the packaging step.