build(handoff)!: switch to LibTL submodule

Removes in-tree Transfer List implementation and updates all references
to use the external LibTL submodule. Updates include paths, Makefile
macros, and platform integration logic to link with LibTL as a static
library.

If you cloned TF-A without the `--recurse-submodules` flag, you can
ensure that this submodule is present by running:

    git submodule update --init --recursive

BREAKING-CHANGE: LibTL is now included in TF-A as a submodule.
  Please run `git submodule update --init --recursive` if you encounter
  issues after migrating to the latest version of TF-A.

Change-Id: I1fa31f7b730066c27985d968698e553b00b07c38
Signed-off-by: Harrison Mutai <harrison.mutai@arm.com>
diff --git a/plat/common/plat_bl_common.c b/plat/common/plat_bl_common.c
index 4772bad..f10f2d7 100644
--- a/plat/common/plat_bl_common.c
+++ b/plat/common/plat_bl_common.c
@@ -9,7 +9,9 @@
 #include <arch_helpers.h>
 #include <common/bl_common.h>
 #include <common/debug.h>
-#include <lib/transfer_list.h>
+#if TRANSFER_LIST
+#include <transfer_list.h>
+#endif
 #include <lib/xlat_tables/xlat_tables_compat.h>
 #include <plat/common/platform.h>
 #include <services/arm_arch_svc.h>