tools/qa-checks: Allow and validate license header format for external works

Externally-licensed works may be included and modified within meta-ewaol. This
patch allows these files to pass the license and copyright header QA-check by
validating an additional alternative format:

    Based on: <Original filename>
    In open-source project: <Source project/repository>
    Original file: Copyright (c) YYYY(-YYYY) <Contributor>
    Modifications: Copyright (c) YYYY(-YYYY) <Contributor>
    SPDX-License-Identifier: <License name>

Original works contributed directly to meta-ewaol continue to use the standard
license and copyright header format:

    Copyright (c) YYYY(-YYYY) <Contributor>
    SPDX-License-Identifier: <License name>

Issue-Id: SCM-3033
Signed-off-by: Richard Neill <richard.neill@arm.com>
Change-Id: Id1b031f29ddb26b3a910365dab9781bd9bd3c4b1