prebuilt: allow running prebuilt images

build

@@ -171,11 +171,7 @@ BR2_ROOTFS_POST_SCRIPT_ARGS=\"${post_script_args}\"
   make O="$buildroot_out_dir" olddefconfig
 fi
 
-mkdir -p \
-  "$gem5_out_dir" \
-  "$qemu_out_dir" \
-  "$p9_dir" \
-;
+common_mkdir
 cd "$buildroot_dir"
 # HOST_QEMU_OPTS is a hack that happens to work because the QEMU package luckly uses += at all times.
 # It shouldn't be necessary in the first place: https://bugs.busybox.net/show_bug.cgi?id=9936