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

Import libfdt v1.4.1 ...

Imports libfdt code from https://git.kernel.org/cgit/utils/dtc/dtc.git
tag "v1.4.1" commit 302fca9.

Change-Id: Ia0d966058beee55a9047e80d8a05bbe4f71d8446