boot: Add BCD store parser

boot: Add BCD store parser

This replaces the memmem-based approach of finding a suitable title
for the windows boot manager with one that actually parses the BCD
store. It's probably faster but more importantly, it's more correct.

The memmem approach may detect stale title strings that are still
in the file but unused due to the way registry hives are updated.
This approach also allows us to detect if the BCD store is multi-boot
so that we can fall back on the generic one instead.