nanobsd: Remove dependency on bsdlabel

The bsdlabel utility is deprecated, gpart should be used instead:

• Offset the first 16 sectors, just like bsdlabel did (used for metadata)
• Use a freebsd-ufs partition type (regardless bsdlabel creating a '!0')

Reviewed by: emaste, imp
Approved by: emaste (mentor)
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D47653