nanobsd: Always define the partition index

Print the desired partition index together with the starting LBA and the
size. Otherwise, when only one code partition is desired, the
configuration partition (/cfg) may not end up in the third slice, thus
breaking the scripts that expect it there.

This was the previous approach, changed in 587c054bea0f ("nanobsd: Use
gpart and create code image before full disk image").

Fix it by printing the index as the last column to reduce the chances of
breaking current overrides, and using it as the gpart index.

PR: 262098
Reviewed by: imp
Approved by: emaste (mentor)
Fixes: 587c054bea0f ("nanobsd: Use gpart and create code image before full disk image")
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D47835

(cherry picked from commit dc4b36b96333f2ab05f3c454f0df2e0a0d4d451d)