add backtrace script and improve config file instructions

See merge request redox-os/redox!1412

Update ci.toml to build required packages for the demo

See merge request redox-os/redox!1411

Remove the rust submodule

Closes #1389

See merge request redox-os/redox!1409

update cookbook

See merge request redox-os/redox!1410

It is no longer necessary for regular builds as of the previous commit.
This will drastically improve the time it takes to do the initial clone
of redox.

The prebuilt tarball already contains it now

Somewhat simplify config.mk

See merge request redox-os/redox!1407

* nproc exists on FreeBSD too
* REDOX_MAKE is generally make, so use that as default with per-OS
  override if necessary.
* HOST_TARGET can be fetched directly from rustc.

Use the standard os-release instead of redox specific redox-release

See merge request redox-os/redox!1402

Rather than just on macOS. This can save quite a bit of space depending
on the amount of free space on the root filesystem of the disk image.

This allows programs not specifically written for Redox OS to discover
information about the OS it is running on.

Clear /tmp when booting

See merge request redox-os/redox!1403

Move all system init configs to /usr/lib/init.d

See merge request redox-os/redox!1404

This keeps /etc/init.d for local overrides without risking them getting
overwritten by system updates.

This matches the behavior of most UNIX systems and ensures that the disk
doesn't eventually fills entirely with unused temporary files.