save_build_config: centralise

config/multithread

@@ -175,6 +175,8 @@ start_multithread_build() {
     ${SCRIPTS}/genbuildplan.py --no-reorder --show-wants --build ${@} > "${THREAD_CONTROL}"/plan || result=1
 
   if [ ${result} -eq 0 ]; then
+    save_build_config
+
     cat "${THREAD_CONTROL}"/plan | awk '{print $1 " " $2}' | \
       MTBUILDSTART=$(date +%s) MTWITHLOCKS=yes ${TOOLCHAIN}/bin/parallel \
         --plain --no-notice --max-procs ${THREADCOUNT} --joblog="${THREAD_CONTROL}/joblog" --plus ${buildopts} \