kboot: Fetch hostfs_root and bootdev from the environment

Fetch bootdev from the environment variable (so it should be set on the
command line). Default to 'zfs:' which will in the future look for the
first zpool that we can boot from. Prior versions of kboot would set
this from the second argument on the command line.

Fetch hostfs_root from the environment (defaulting to '/'). Prior
versions of kboot would set this from the first arg on the command line.

Sponsored by: Netflix
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D38010