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>

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.