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.

tools: Added astyle,pylint and xmllint checks

Added small bash scripts to run astyle, pylint and xmllint
checks over the kernel source. These style checks were ported
from the old Make build system.