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>
19 files changed