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.

hardware_gen: add elfloader CPUs output

outputs all CPUs described in the DT to the elfloader
header and also includes any devices in the seL4,elfoader-devices
property.

tools: rewrite hardware_gen.py

This is almost a complete rewrite from the old hardware_gen.py.

It separates the 'parse DT' stage from the 'generate output'
devices more strictly, and is hopefully easier to understand and
easier to extend.

We also no longer generate the 'devices' list (in YAML)
or the dev_p_regs array (in C), as the kernel will implicitly
expose all non-RAM untypeds as devices.