This adds support for importing this project via `find_package(seL4)` if
the project exists in the CMAKE_MODULE_PATH.
sel4_import_kernel adds the kernel project to the current build
directory.
sel4_import_libsel4 adds libsel4 to the current build directory
sel4_configure_platform_settings imports seL4Config.cmake which
configures kernel configuration and toolchain settings for the target
platform.
