bootimage: Derive sunlabel(8) input from the disklabel protofile

For USE_SUNLABEL labeling, stop hard-coding the sunlabel(8) geometry
in cylinders in Makefile.bootimage. Instead, generate the sunlabel(8)
command stream by parsing the generated disklabel protofile with
a small awk helper.

This keeps the Sun disk label consistent with the disklabel(8) protofile
and avoids maintaining two independent sets of geometry/partition logic.

Tested with a sun3 live-image on TME.