Convert license tags to SPDX identifiers

This commit also converts our own copyright headers to directly use
SPDX, but leaves all other copyright header intact, only adding the
SPDX ident. As far as possible this commit also merges multiple
Data61 copyright statements/headers into one for consistency.

Add missing license to tools/python-deps/README.rst

Add pip metapackage for downloading dependencies

Dependencies can now be downloaded with:
pip install sel4-deps

Dependencies can be changed by modifying the setup.py script.

Information on how to update the package is in the setup.py script.

JIRA: SELFOUR-909