merge the rest of sbmips into evbmips. install the stand files in
/usr/mdec/sbmips/ and document them in installboot(8) even though
it isn't really ported yet. also build the SBMIPS kernels.

unfortunately, sbmips has been broken since mips64 merge, but the
fix can happen in the evbmips version, which we might merge into
the older trees anyway.

XXX: releng can turn off sbmips* builds now for -current.