loader: lua: remove the default kernel if it doesn't exist

The kernel env var provides the default kernel, usually "kernel". It
may be the case that the user doesn't have a "kernel" kernel, just
"kernel.*" kernels, but have left kernel to the default because we
autodetect entries by default anyways.

If we're doing autodetection, take note of whether the default kernel
exists or not and remove it from the list if it doesn't and we had found
any other kernels. We avoid it in the #kernels == 1 case because
something fishy has likely happened and we should just trust the
configuration.

Reviewed by: imp, manu
Differential Revision: https://reviews.freebsd.org/D42967