scripts: Add SPDX checker script

The SPDX-License-Identifiers are growing in the kernel and so grow
expression failures and license IDs are used which have no corresponding
license text file in the LICENSES directory.

Add a script which gathers information from the LICENSES directory,
i.e. the various tags in the licenses and exception files and then scans
either input from stdin, which it treats as a single file or if started
without arguments it scans the full kernel tree.

It checks whether the license expression syntax is correct and also
validates whether the license identifiers used in the expressions are
available in the LICENSES files.

scripts/spdxcheck.py -h
usage: spdxcheck.py [-h] [-m MAXLINES] [-v] [path [path ...]]

SPDX expression checker

positional arguments:
  path                  Check path or file. If not given full git tree scan.
                        For stdin use "-"

optional arguments:
  -h, --help            show this help message and exit
  -m MAXLINES, --maxlines MAXLINES
 ...