lmp/bb-build: only use the sstate-cache mirror on the first step

On the fisrt build step we run bitbake for setscene only tasks
and obvious this one is the only one that use the reads and use the
sstate-cache.
The second step  will be a 0% hit of available because everything
has already been accelerated in the first step. In this second step
everything will be build from scratch and the local sstate-cache
populadted with the artifacts.

This 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

@@ -77,6 +77,8 @@ 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