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>
diff --git a/.gitmodules b/.gitmodules
new file mode 100644
index 0000000..858e0c1
--- /dev/null
+++ b/.gitmodules
@@ -0,0 +1,4 @@
+[submodule "libtl"]
+	path = contrib/libtl
+	url = https://review.trustedfirmware.org/shared/transfer-list-library
+	shallow = true