Make NanoBSD source-able from other scripts.

Summary:
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.

Reviewers: imp

Reviewed By: imp

Differential Revision: https://reviews.freebsd.org/D1321