Convert to SPDX license tags

This includes marking all documentation files CC-BY-SA-4.0.

Move object_sizes to capdl

This defines a custom target object_sizes which preprocesses a YAML file
to contain seL4 object size constants that can then be imported by
python-capdl-tool. This was previously located in the camkes-tool
repository but is moved here as it is more specific to capdl than
Camkes.

CMake: Add toplevel CMakeLists.txt

This calls add_subdirectory on the capdl-loader-app cmake project