feat(handoff): support libtl submodule builds

Refactor transfer list support to enable building the transfer list
and updates include paths accordingly.

Change-Id: Icdbe19924678a4023c15897a9765b8e7f150d9e3
Signed-off-by: Harrison Mutai <harrison.mutai@arm.com>

.gitmodules

@@ -0,0 +1,4 @@
+[submodule "libtl"]
+	path = contrib/libtl
+	url = https://review.trustedfirmware.org/shared/transfer-list-library
+	shallow = true

contrib/libtl

@@ -0,0 +1 @@
+Subproject commit 9630cb01984503a9fe6974d31b0d581fb693540c

docs/getting_started/prerequisites.rst

@@ -182,6 +182,44 @@ manually by running:
 You can read more about Git hooks in the *githooks* page of the Git
 documentation, available `here <https://git-scm.com/docs/githooks>`_.
 
+.. _git_submodules:
+
+Cloning Additional Git Submodules
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Some dependencies in TF-A, such as Transfer List Library ``libtl``, are managed
+using Git submodules. Submodules allow external repositories to be included
+within the main project while maintaining their own commit history.
+
+Initial Clone with Submodules
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+If you're cloning the repository for the first time, run the following commands
+to initialize and fetch all submodules:
+
+.. code-block:: bash
+
+   git clone --recurse-submodules "https://git.trustedfirmware.org/TF-A/trusted-firmware-a"
+
+This ensures all submodules (including ``libtl``) are correctly checked out.
+
+Updating Submodules
+^^^^^^^^^^^^^^^^^^^
+
+If the project updates the reference to a submodule (e.g., points to a new
+commit of ``libtl``), you can update your local copy by running:
+
+.. code-block:: bash
+
+   git pull
+   git submodule update --init --recursive
+
+To fetch the latest commits from all submodules, you can use:
+
+.. code-block:: bash
+
+   git submodule update --remote
+
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