update license information in README

* Point to `LICENSES/` and include the syscall note in the README.
* BSD and GPL license files now provided in LICENSES directory.

dep5 file for additional license information

This file provides license and copyright information for everything
that cannot be tagged inline.

add a license for .licenseignore

For completeness.

Manually tag missing 3rd-party licenses

Two files were previously ignored in the license check.

licenseignore: generated file

references.bib is in version control for deterministic builds, but
it is a generated file.

.gitignore has a license

The .gitignore file doesn't need to be removed from the license check.

licenseignore: don't ignore the manual

The software license doesn't make much sense for the manual, but
it is for now the only one available.

RISCV: Remove hartid and DTB parsing from the boot code.

riscv: fix licenses

- add files from elsewhere to the license ignore
- s/D61_BSD/DATA61_BSD

Add CHANGES to license ignore

Add .licenseignore for seL4

This tells the license tool what files to avoid checking for licenses