Make NanoBSD source-able from other scripts.
This change converts NanoBSD into a two-script bundle.
- defaults.sh contains all non-CLI code. Most NanoBSD code is moved into this file.
- nanobsd.sh now consists just of a command line interface that calls into functions in defaults.sh.
Test Plan: Run NanoBSD using a previously-working configuration.
Reviewed By: imp
Differential Revision: https://reviews.freebsd.org/D1321