Convert license tags to SPDX

Also clarifies provenance of code in `apps/fdtgen/fdt_utils.c`

Signed-off-by: Gerwin Klein <gerwin.klein@data61.csiro.au>

CMake: Apply Kernel configuration in setting stage

We aim to run all settings.cmake files before the kernel's CMake scripts
are processed. This is in order to reach a stable Config cache within a
single invocation of cmake.

trivial: Fix style for recently changed files

CMake: Fail if incorrect arch selected

Forcing the architecture to the only supported architecture causes
confusing build output, as by the time that these CMakeLists.txt files
are processed, the architecture is immutable. Failing with an error
message is more helpful.

Update remainder apps to new camkes proj structure

Port example CAmkES applications to cmake

Adds a cmake build definition for each of the 'apps' and provides a small wrapper
CMakeLists.txt for setting common options and picking the application to build.