envkernel.py: don't search for kbuild_out if _outdir is set

Patch by Oliver, fixes #2000 (closed).

Tested by compiling a couple of kernels that uses _outdir, and one without. pmbootstrap build --envkernel linux-{} works for all of them.

Most downstream kernels seem to need a few patches to silence (or fix) warnings when makeing with *-musl-gcc under envkernel though, but that is an unrelated issue.