feat(handoff): introduce firmware handoff library

Add transfer list APIs and firmware handoff build option.

Change-Id: I68a0ace22c7e50fcdacd101eb76b271d7b76d8ff
Signed-off-by: Raymond Mao <raymond.mao@linaro.org>
diff --git a/changelog.yaml b/changelog.yaml
index cdbedbb..0d69fb3 100644
--- a/changelog.yaml
+++ b/changelog.yaml
@@ -784,6 +784,9 @@
       - title: Semihosting
         scope: semihosting
 
+      - title: Firmware Handoff
+        scope: handoff
+
   - title: Drivers
 
     subsections: