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.

hardware_gen: add elfloader output

The elfloader is going to start using devices based on the device tree.
Add an output method for hardware_gen.py that generates header files
that the elfloader can use. Currently they contain an array of
"devices", where each device has a compatiblity string and a series of
regions (represented simply as the physical address of the region).

For now, the elfloader only uses the serial device specified by the
stdout-path property in the DTB.