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

move DTS to the kernel

dts: Suppress output unless error

Some dts compilations generate a lot of output everytime CMake is run.

cmake: add helper to generate DTB from DTS

cmake: add FindDTS to locate dts files