circleci: make logs of build failures more readable

Due to concurrent build, it's currently not that easy to find the proper
reason for the package build failure, so let's just build the failing
package with -j1 in order to provide more human readable build log
failures.

Ref: https://github.com/openwrt/packages/pull/9584#issuecomment-518038631
Signed-off-by: Petr Štetiar <ynezz@true.cz>

.circleci/config.yml

@@ -143,7 +143,11 @@ jobs:
 
              for PKG in $PKGS ; do
                  echo_blue "===+ Building: $PKG"
-                 make "package/$PKG/compile" -j3 V=s
+                 make "package/$PKG/compile" -j3 V=s || {
+                        RET=$?
+                        echo_red "===+ Building: $PKG failed, rebuilding with -j1 for human readable error log"
+                        make "package/$PKG/compile" -j1 V=s; exit $RET
+                 }
              done
 
       - store_artifacts: