lmp/bb-build: only use the sstate-cache mirror when it is necessary

Our build can be divided into two steps:
1 - build only using the sstate-cache
2 - build everything else missing

On the step [1] we run bitbake for setscene only tasks
and obvious this one is the only one that uses and reads the
sstate-cache mirror.

The step [2] will have 0% hit of cache available because everything
has already been accelerated in the step [1]. In the step [2]
everything will be build from scratch and the local sstate-cache
will be populated with the new artifacts.

This change will improve and significantly reduce the load generated on
the sstate-mirror http server by our CI builds.

Signed-off-by: Jose Quaresma <jose.quaresma@foundries.io>

Author: quaresmajose

lmp/bb-build.sh

@@ -78,6 +78,8 @@ fi
 # Setscene (cache), failures not critical
 status "Run bitbake (setscene tasks only)"
 bitbake -DD --setscene-only --continue ${IMAGE} || true
+# we have now everything from the mirror so we don't need that anymore
+echo 'SSTATE_MIRRORS = ""' >> conf/local.conf
 
 # add trap to do some pending operations on exit
 trap finish TERM INT EXIT