scripts/spdxcheck: Put excluded files and directories into a separate file

The files and directories which are excluded from scanning are currently
hard coded in the script. That's not maintainable and not accessible for
external tools.

Move the files and directories which should be excluded into a file.  The
default file is scripts/spdxexclude. This can be overridden with the
'-e $FILE' command line option.

The file format and syntax is similar to the .gitignore file.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>