api.payload: Add dlc support to api

We needed some changes to gspaths and the endpoint so we can
successfully generate a DLC image payload. This commit implements this.

BUG=chromium:1122854
TEST=new tests

Change-Id: Ibdb150ac8641707c52c11103c1025fb96ac8525d
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2547501
Reviewed-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Navil Perez <navil@google.com>
Commit-Queue: George Engelbrecht <engeg@google.com>
Tested-by: George Engelbrecht <engeg@google.com>
Auto-Submit: George Engelbrecht <engeg@google.com>
diff --git a/api/controller/payload.py b/api/controller/payload.py
index a32ca36..0195318 100644
--- a/api/controller/payload.py
+++ b/api/controller/payload.py
@@ -16,8 +16,10 @@
 
 _VALID_IMAGE_PAIRS = (('src_signed_image', 'tgt_signed_image'),
                       ('src_unsigned_image', 'tgt_unsigned_image'),
+                      ('src_dlc_image', 'tgt_dlc_image'),
                       ('full_update', 'tgt_unsigned_image'),
-                      ('full_update', 'tgt_signed_image'))
+                      ('full_update', 'tgt_signed_image'),
+                      ('full_update', 'tgt_dlc_image'))
 
 _DEFAULT_PAYGEN_CACHE_DIR = '.paygen_cache'