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.

Add a CMake based build system

This commit adds an alternate build system using CMake that operates indepenently of
the existing Kconfig+Kbuild+make based build system