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update bootstrap.sh for macOS brew using steps from The Book

See merge request redox-os/redox!1231

Install cargo-config from bootstrap script

See merge request !1230

Some cookbook entries were failing with an error like this:

```
error: no such subcommand: `config`
```

We can install this subcommand from the bootstrap script, and also
instruct folks in the README to do so manually.

This reverts commit c5d3a92a.

This reverts commit 8cccf0ed.

Fix .partial paths

Closes #1283

See merge request !1229

Fixes #1283

Add optional and backwards-compatible MBR support in the bootloader

See merge request !1227