Add locking to a few scripts

Prevent scripts from being run while they're still being run
by headless setup.

Run with read-only boot and root

The presence of the one-step setup configuration file would trigger the
boot and root partitions to be remounted read-write again on every boot.
This change makes rc.local move rather than copy the setup file from boot
to root, and makes the setup_progress logging function handle a read-only
log file.

Update README.md

Reorganize pi-gen-sources, add documentation

Add instructions for building an image, move some files around to make
the process simpler.
Add btrfs-tools since I plan to use those in the future and don't
want to have to make a new image then.
Also updated a few references in docs and scripts from cimryan to
therealmarcone, since this fork no longer works with the scripts
and instructions from cimryan.