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.

python2 --> python3

Update all scripts and build system to call python3, given python2's
upcoming doom. Use sys.maxsize instead of sys.maxint in one script
(maxint does not exist in python3).

style: run autopep8 on python files

tools: Ignore option for circular_includes script

Added an "ignore" argument to the circular_includes script. This
allows the caller to specify files for the script to ignore when
it parses the source file. Rather than creating a special
ignore case for "kernel_all.c" in the script itself, the user
parses the file as an argument (plus others if needed). Updated
the kernels cmake file to reflect the change.

tools: fix licenses

tools: Make circular_includes check more robust

Allows for an arbitrary path to be applied to kernel_all.c in case the build is not
happening in the current working directory

Replace circular includes perl script with python