http_prebuilt: Implement archive of artifacts

* checksum will be downloaded and asserted to prevent a
    redownload every clean
* Archives will exist (invisibly) at vendor/cm/prebuilt/archive

Change-Id: I637e86cacd146d665044a119885900dc4660e22b
Signed-off-by: Brandon McAnsh <brandon.mcansh@gmail.com>
diff --git a/.gitignore b/.gitignore
index f5b82f3..931ce07 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1 +1,2 @@
 proprietary/*
+prebuilt/archive