Default to softraid on boot-device iff found

Default to softraid on boot-device iff found

root on softraid on sparc64 currently requires explicitly passing the volume
name as part of the boot arguments, e.g.
	boot-file=sr0
	boot-file=sr0a:/bsd
or
	{ok} boot my-devalias sr0:

(ofwboot always probes for softraid devices, assembles them but continues
 to ignore such volumes unless specified.)

This is inconsistent with softraid support on other platforms and has
further drawbacks when it comes to sysupgrade logic.

Unless an explicit root disk was given, make ofwboot default to the softraid
volume on the boot device to make root on softraid work out of the box
without having to set variables or pass arguments in OpenBoot.

Eventually, ofwboot's disk discovery and softraid assembly machinery will be
fixed and boot-file won't be misused for softraid purposes anymore.

Remove the quirky softraid paragraph from boot_sparc64(8) now that it
is purely optional, expecting users from now on to either leave boot-file
empty or pass a kernel filename alone.

Tested on T4-2 guest domains with and without root on softraid.

Feedback claudio kettenis stsp
OK kettenis (previous diff)
OK stsp