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: add a README

This readme explains the basic flow of the elfloader on
ARM and RISC-V. It also explains the various image types
supported on ARM, and provides an overview of the driver framework.