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.

mcs: scheduling context donation over ipc

After this commit, threads blocked on an endpoint can recieve a
scheduling context from the thread that wakes the blocked thread.

python2 --> python3

Update all scripts and build system to call python3, given python2's
upcoming doom. Use sys.maxsize instead of sys.maxint in one script
(maxint does not exist in python3).

manual: use level 3 for syscalls

As syscalls are not part of an inner group, which bumps the level,
they need to start at 3 not 2

manual: promote sel4_arch API docs level

This change makes sel4_arch api docs the same level as the base arch api docs. This is because the
header level became too deep for latex section types, and there is not much advantage to such deep
hierarchies.

manual: use int level in parse_doxygen_xml.py

The level argument to parse_doxygen_xml.py was previously a latex section header (either subsection
or subsubsection). This change converts the argument to numerical which is more robust and is not
limited to 2 specific levels.

This change
- alters the level argument to be an int, where 0 = highest level header,
- removes the translation of latex header to markdown,
- adds a new 'level_to_header' function to the generator class, which translates the level number
to a header for that specific output type.

manual: Markdown generation rule in Makefile

Added support to generate the libsel4 api in markdown within the
manual Makefile.

riscv: document API methods

manual: Expand parts of the x86 virtualisation documentation

This provides documentation for kernel design of the x86 virtualisation, the additional
syscall and object invocations. The ARM design is not fully expanded.

manual: Variable for python interpreter

This allows the manual to be built with `PYTHON=mypython make`, to
invoke the python scripts involved in generating documentation with a
particular version of python. The scripts are compatible with both
python2 and python3, but this should simplify debugging if this ever
stops being the case. If no value of PYTHON is specified, the default
value of "python" is used.

manual: Suppress verbose output when building

manual: Docs for debugg and benchmark syscalls

manual: Validate interface files against dtd

manual: Generate sections for arch-specific calls

This just creates the sections with appropriate names and function
prototypes, with no textual description of what the functions do.

manual: Generating tex from obj inv header

JIRA: SELFOUR-606

manual: More verbose Makefile printouts

JIRA: SELFOUR-606

manual: Suppressing doxygen warnings

JIRA: SELFOUR-606

manual: Tex filenames derived from doxy filenames

Rather than having a custom naming convention for generated latex files,
we use names based on the names given to xml files generated by doxygen,
which in turn take their names from groups defined in doxygen comments
in various header files.

JIRA: SELFOUR-606

Syscalls docs in manual generated by doxygen

This adds a dummy header with syscall prototypes with doxygen comments.
The build process for the manual includes parsing these comments using
doxygen. Doxygen produces an xml file which is parsed by a script to
generate latex which is included in the manual. This approach was chosen
over doxygen's native latex output to gain greater control over the
formatting of the generated documentation, and to take advantage of
existing api-formatting support in the sel4 manual.

Related issue: SELFOUR-606

Release snapshot