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.

Rename shared_types*.bf files to be consistent

Now these bitfield definitions are generated from the same source file
in the kernel and libsel4, and also result in the same include path: <sel4/shared_types_gen.h>