Add SSTATE_DIR and DL_DIR configuration

ci/conf/ci.conf

 # === SETTINGS FOR PROJECT OR SOURCECODE ===
 
+# Where to save build caches
+export SSTATE_DIR="${HOME}/yocto-common/rocko/sstate-cache"
+export DL_DIR="${HOME}/yocto-common/rocko/downloads"
+
 # Build directory name
 # The build branch has to be modified for every branch. gitlab looses
 # its branch tag when doing a ci build. So we have to set it manually