Convert license tags to SPDX identifiers

This commit also converts our own copyright headers to directly use
SPDX, but leaves all other copyright header intact, only adding the
SPDX ident. As far as possible this commit also merges multiple
Data61 copyright statements/headers into one for consistency.

CMake: Add FindseL4.cmake module

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.