nanobsd: Offset code partition to a track boundary

[jlduran]

Summary

Ensure the primary and secondary code partitions start at a proper track
boundary by applying a NANO_SECTS offset in bytes.

[imp]

With the proviso that track boundaries are almost useless today, even for MBR which once upon a time was tied to track boundaries.

[jlduran]

In D57221#1311207, @imp wrote:

With the proviso that track boundaries are almost useless today, even for MBR which once upon a time was tied to track boundaries.

Stay tuned! I'll add a GSoC teaser to the commit message also indicating this fact is soon to be updated as well.