ostro.conf: revise swupd content reuse

The previous approach reused content from the latest build. The goal
was to make the build as fast as possible by maximizing reuse.

However, current Ostro OS release procedures do not guarantee that the
"builds" directory is kept around and therefore reusing content from
it is not currently possible. These procedures could be changed, but
until then we have to use the "milestone" directory.

Signed-off-by: Patrick Ohly <patrick.ohly@intel.com>

Author: pohly

meta-ostro/conf/distro/ostro.conf

@@ -29,9 +29,12 @@ OSTRO_SDK_UPDATE_URL ??= "https://download.ostroproject.org/builds/ostro-os/late
 #
 # The version URL determines what the client picks as the version that it updates to.
 # The content URL must have all builds ever produced and is expected to also
-# have the corresponding version information.
+# have the corresponding version information. In Ostro OS, only content published
+# as milestone is guranteed to be preserved. Therefore we cannot use the build
+# artifacts from the previous build and have to fall back to the much older previous
+# milestone, which is less efficient.
 SWUPD_VERSION_URL ?= "https://download.ostroproject.org/updates/ostro-os/milestone/${MACHINE}/${SWUPD_IMAGE_PN}"
-SWUPD_CONTENT_URL ?= "https://download.ostroproject.org/updates/ostro-os/builds/${MACHINE}/${SWUPD_IMAGE_PN}"
+SWUPD_CONTENT_URL ?= "https://download.ostroproject.org/updates/ostro-os/milestone/${MACHINE}/${SWUPD_IMAGE_PN}"
 
 # Must match the certificate used by the version and content server.
 SWUPD_PINNED_PUBKEY ?= "${META_OSTRO_BASE}/files/ostroprojectorg.key"