build(measured-boot)!: move to ext event log lib

Removes in-tree Event Log library implementation and updates all
references to use the external submodule. Updates include paths,
Makefile macros, and platform integration logic to link with lib as a
static library. Some of the event log utilities related to firmware
handoff have been moved into that library, accordingly bump the version
of LibTL.

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

    git submodule update --init --recursive

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

Change-Id: I5c681ab7621c8bcdfc06793a81781af3439964a6
Signed-off-by: Harrison Mutai <harrison.mutai@arm.com>
diff --git a/.gitmodules b/.gitmodules
index c753963..844ca3c 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -5,3 +5,6 @@
 	path = contrib/libtl
 	url = https://review.trustedfirmware.org/shared/transfer-list-library
 	shallow = true
+[submodule "contrib/libeventlog"]
+	path = contrib/libeventlog
+	url = https://review.trustedfirmware.org/shared/libEventLog