Licence Checker: Support Relicencing

[lenary]

OpenTitan uses some headers of the format (this from util/opentitan-pgm-fpga/vivado_pgm.tcl)

# Copyright lowRISC contributors
# Copyright <other> contributors
# Licensed under the 2-Clause BSD License, see LICENSE.edalize for details.
# SPDX-License-Identifier: BSD-2-Clause

We should support this in the licence checker, rather than adding this kind of file to the exclusion list.

[asb]

I think there are benefits to having to explicitly add an exclusion for code that isn't under the standard license, and suspect we'd want to maintain that behaviour for OpenTitan.

If moving to accept by default, it would be good to also support listing all files that aren't under the standard "primary" license.

[lenary]

The idea is not accept by default, the idea is to accept exactly the above format as also allowed (modulo the contributor name). Any other formats would still be required to go into the exclusion list.

We can always print out a list of those not matching the standard, primary licence, but for which we think have a correct header.

[asb]

I meant it would move to accepting this format by default (which is how I understood the issue). Any import of third-party IP should be manually verified, and so adding an entry to the exclusion list as we do now seems the right workflow.