Sparc miniroot hasn't worked for a while, since the sed code to find

Sparc miniroot hasn't worked for a while, since the sed code to find
the root disk was broken. Simplify and fix said sed code. Problem
noted by okan@ and others, diagnosed by deraadt@, sed suggestions from
nordin@, jmc@, fgsch@, guenther@.

'get something in before sparc gets done' deraadt@