Configure hostname in headless mode, add retry

setup-teslausb:
Always configure the host name, even in headless setup, and set the
specified name early on during setup, so the device is reachable by its
new host name during early setup.

prebuilt image sources:
Set the hostname when configuring wifi, so it's effective as soon as
wifi comes up.
Retry the setup-teslausb script download until it succeeds, since it
sometimes runs before the network is available and then fails.
Remove btrfs-tools and add xfsprogs to the release image packages.

Fix check for setup conf file

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.