Fix -r on multi-chunk softraid volumes

Fix -r on multi-chunk softraid volumes

Running installboot(8) on softraid(4) volumes means installing stages on
every softraid chunk.

The overall idea is the same, but MD implementations differ.

sparc64_softraid.c's sr_install_bootblk() reuses sparc64_installboot.c's
md_installboot() for this.

For sparc64, md_installboot() does the copy of stage 2, usually
/usr/mdec/ofwboot to /ofwboot, so when `-r root' is passed, it prefixes the
file path with "root".

For single-disk installations (plain-disk and single-chunk softraid) this is
fine, but as soon as multiple chunks are used, md_installboot() currently
prefixes the path each time, obviously resulting in invalid paths starting
with the second run.

Other architectures do reuse md_installboot() as well but either don't do
such a copy or implement the prefixing differently -- plus they must support
softraid in the firt place to be able to hit this type of bug.

With this fixed, regress/usr.sbin/installboot finally passes on sparc64 and
installboot no longer fails at the end of a fresh installation onto softraid
with multiple chunks.

"looks correct" miod