feat: add logging infra w/ backend support

Add functions to support logging. This enables a user specifying their
own logging format for all functions. TF-A's logging format is supported
out of the box by specifying the `DEBUG_BACKEND_HEADER` as
`log_backed_tf.h`.

Implement digest helpers that avoid heap/libc as some end projects don't
have access to this.

Change-Id: I55410ab963ea4d0c1956ba41f94b67eced8621d5
Signed-off-by: Harrison Mutai <harrison.mutai@arm.com>
diff --git a/include/event_log.h b/include/event_log.h
index f97c580..ee54f9a 100644
--- a/include/event_log.h
+++ b/include/event_log.h
@@ -37,6 +37,12 @@
 	size_t count;
 };
 
+struct event_log_algorithm {
+	uint16_t id;
+	const char *name;
+	uint16_t size;
+};
+
 #define ID_EVENT_SIZE                                           \
 	(sizeof(id_event_headers_t) +                           \
 	 (sizeof(id_event_algorithm_size_t) * HASH_ALG_COUNT) + \