Translate the Linux error return from read to a FreeBSD errno. We use a
simplified translation: 1-34 are the same between the systems, so any of
those will be returned directly. All other errno map to EINVAL. This
will suffice for some code that reads /dev/mem in producing the right
diagnostic.
A fully generalized version is much harder. Linux has a number of errno
that don't translate well and has architecture dependent
encodings. Avoid this mess with a simple macro for now. Add comment
explaining why we use the simple method we do.
Sponsored by: Netflix
Reviewed by: kevans, andrew
Differential Revision: https://reviews.freebsd.org/D38265
Add hostfs for the Linux environment. We can't use the userboot one
that's kinda similar because the Linux system calls we have in kboot are
not quite POSIX compliant (Linux takes care of providing the POSIX
interface in libc), so we have to cope with a number of quirks in that
area.
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D36607
