Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

sparc64/zfs boot: take advantage of new libzfsboot capabilities

Also drop the now unneeded compatibility shims.

Tested by: marius
MFC after: 1 month

MF tbemd: move to using specific architecture makefiles