nanobsd: Use rounded sizes for cfg and data slices

[jlduran]

Summary

Use the calculated (rounded up) cfg and data sizes from the
_.partitioning file, instead of consuming them directly from the global
variables. We obtain the size of the cfg and data slices by explicitly
searching for index 3 and 4 respectively in the _.partitioning file.

This ensures that the final image has the rounded-up sizes, and not the
raw sizes.