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.

CMake: expose hook for enabling ElfloaderRootserversLast

This allows the parent build system (typically CAmkES) to enable this
setting before the C configuration is generated from config_option.