Rework bootparttest to use more code from sys/boot.
Use disk_open() call to emulate loader behavior.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272557 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
Fix typo.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272558 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f