Move stdlib header files to include/lib/stdlib

* Move stdlib header files from include/stdlib to include/lib/stdlib for
  consistency with other library headers.
* Fix checkpatch paths to continue excluding stdlib files.
* Create stdlib.mk to define the stdlib source files and include directories.
* Include stdlib.mk from the top level Makefile.
* Update stdlib header path in the fip_create Makefile.
* Update porting-guide.md with the new paths.

Change-Id: Ia92c2dc572e9efb54a783e306b5ceb2ce24d27fa
diff --git a/tools/fip_create/Makefile b/tools/fip_create/Makefile
index 2e367c2..30e4b82 100644
--- a/tools/fip_create/Makefile
+++ b/tools/fip_create/Makefile
@@ -69,7 +69,7 @@
 # path. This avoids conflicts with definitions in the compiler standard
 # include path.
 #
-uuid.h : ../../include/stdlib/sys/uuid.h
+uuid.h : ../../include/lib/stdlib/sys/uuid.h
 	$(call SHELL_COPY,$<,$@)
 
 firmware_image_package.h : ../../include/common/firmware_image_package.h