replacing all ifndef with pargma once

All the kernel header files now use pargma once rather than the ifndef,
as the pre-processed C files do not change while header files
are protected with pargma once. This will also solve any naming issues
caused by ifndef.

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.

Expose string functions in all builds

These are useful beyond just debug and printing builds

Correct separation of printing and debug builds

Fixes some build issues with 541289a32603cee8242b5360b05e8f0c52795433
as well as further allowing debugging (via the capdl interface) to
happen when printing is turned off.

unsigned int -> word_t

Add ability to set a human readable thread name when running kernel in debug mode