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: Refactor helpers to be included on demand

As more helper files get added it becomes more expensive to include them
all in common.cmake. We instead add them to the CMAKE_MODULE_PATH and
allow them to be imported as CMake modules. Adding global include guards
allows them to be imported multiple times without processing every time.

style: style cmake files consistently

cmake-tool: Move default riscv-pk search to tools

RISC-V: Add CMake support

cmake: Sort projects for deterministic builds

Add a common CMake based build tool/system

For details read the added cmake-tool/README.md