bhnd(4): Use a stable sort key to produce deterministic nvram_map_gen.awk
output.

When ordering SROM layout entries, we now use the unique (var_id,
rev_start, rev_end) tuple as the sort key; this fixes the previously
non-deterministic output when sorting entries with overlapping var_ids.

PR: 215422
Reported by: emaste
Reviewed by: emaste
Approved by: adrian (mentor)
Differential Revision: https://reviews.freebsd.org/D8859