nanobsd: Get the code size by its partition index

[jlduran]

Summary

Previously the code was assumed to be on the on the first line of the
_.partitioning file. Instead, explicitly look up the size by its
partition index to make the parsing order-independent.

The _.partitioning file:

1. First column: starting sector.
2. Second column: size in 512-byte sectors.
3. Third column: partition index.

Get the code size by explicitly selecting when the partition index is 1.