trivial: Change ELF library to BSD 3 Clause

Signed-off-by: Curtis Millar <c.millar@unsw.edu.au>

convert license headers to SPDX

Includes license review of 3rd-party files, adding missing upstream
headers, and moving the UNSW OZPLB license into its own file rather
than replicating it in the header.

elfloader: refactor files to imitate kernel design
- move files to src/include folders
- update files as required, e.g. renaming files and folders to prevent conflicts