Improve SPDX identifier coverage (#716)

* Change MIT license texts to SPDX-License-Identifier

SPDX-License-Identifier is much easier to understand and grep than
license text so use that instead.

* Change GPL exception license texts to SPDX-License-Identifier

SPDX-License-Identifier is much easier to understand and grep than
license text so use that instead.

* Change misc license texts to SPDX-License-Identifier

There are some external code in repo which are not licenses as most of
the stuff in this repo. We still want every file to have SPDX identifier
to easily grep licenses.

* Add currently used license files

Even though Bacnet-Stack is using SPDX identifiers we still need to give
those license files with source. For this reason add all license files
to license/ folder.

SPDX has also files which would make same thing but this is style which
example Linux kernel is using and it is quite clear so I choose that one
for now.

I choosed not yet bring CC-PDDC as that is not right license for those
files.

---------

Co-authored-by: Kari Argillander <kari.argillander@fidelix.com>

ports/bdk-atxx4-mstp/led.h

@@ -2,24 +2,8 @@
 *
 * Copyright (C) 2009 Steve Karg <skarg@users.sourceforge.net>
 *
-* Permission is hereby granted, free of charge, to any person obtaining
-* a copy of this software and associated documentation files (the
-* "Software"), to deal in the Software without restriction, including
-* without limitation the rights to use, copy, modify, merge, publish,
-* distribute, sublicense, and/or sell copies of the Software, and to
-* permit persons to whom the Software is furnished to do so, subject to
-* the following conditions:
+* SPDX-License-Identifier: MIT
 *
-* The above copyright notice and this permission notice shall be included
-* in all copies or substantial portions of the Software.
-*
-* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
-* EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
-* MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
-* IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
-* CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
-* TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
-* SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 *********************************************************************/
 #ifndef LED_H
 #define LED_H