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>

README.md

@@ -51,12 +51,8 @@ skeleton or example or template files, and are not copyrighted as GPL.
 
 The text of the GPL exception included in each source file is as follows: 
 
-"As a special exception, if other files instantiate templates or use macros or
-inline functions from this file, or you compile this file and link it with
-other works to produce a work based on this file, this file does not by itself
-cause the resulting work to be covered by the GNU General Public License.
-However the source code for this file must still be made available in
-accordance with section (3) of the GNU General Public License."
+ * SPDX-License-Identifier: GPL-2.0-or-later WITH GCC-exception-2.0
+                                             ^^^^^^^^^^^^^^^^^^^^^^
 
 The code is written in C for portability, and includes unit tests (PC based
 unit tests). Since the code is designed to be portable, it compiles with GCC as