Use internal stdint and stddef

This changes sel4runtime to use the internal minimal stdint and stddef
removing the dependency for those files from an external libc.

This also removes the imports and dependencies for stdbool, string, and
assert.

Signed-off-by: Curtis Millar <curtis@curtism.me>

trivial: style fix

Signed-off-by: Curtis Millar <curtis@curtism.me>

Move env init into external includes

This allows an external runtime to re-use the initialisation of the
runtime environment.

Signed-off-by: Curtis Millar <curtis@curtism.me>

Update licence dates.

Fix duplication of NULL and constness.

Store more of the runtime state.

Load environment into runtime.

Fix indentation.

Add licenses to all files.

Rename from sel4rt to sel4runtime

Change C entrypoint name

Change the C entrypoint name to match the entrypoint name for root
tasks.