Use SPDX license identifiers ...

To make software license auditing simpler, use SPDX[0] license
identifiers instead of duplicating the license text in every file.

NOTE: Files that have been imported by FreeBSD have not been modified.

[0]: https://spdx.org/

Change-Id: I80a00e1f641b8cc075ca5a95b10607ed9ed8761a
Signed-off-by: dp-arm <dimitris.papastamos@arm.com>

Minor libfdt changes to enable TF integration ...

* Move libfdt API headers to include/lib/libfdt
* Add libfdt.mk helper makefile
* Remove unused libfdt files
* Minor changes to fdt.h and libfdt.h to make them C99 compliant

Co-Authored-By: Jens Wiklander <jens.wiklander@linaro.org>

Change-Id: I425842c2b111dcd5fb6908cc698064de4f77220e