Allow systemd to run more than one service on usb mount udev event

Signed-off-by: Vincent Prince <vincent.prince.fr@gmail.com>

recipes-support/swupdate/swupdate/swupdate-usb.rules

@@ -1,2 +1,2 @@
-ACTION=="add", KERNEL=="sd*", SUBSYSTEM=="block", ENV{ID_FS_USAGE}=="filesystem", TAG+="systemd", ENV{SYSTEMD_WANTS}="swupdate-usb@%k.service"
+ACTION=="add", KERNEL=="sd*", SUBSYSTEM=="block", ENV{ID_FS_USAGE}=="filesystem", TAG+="systemd", ENV{SYSTEMD_WANTS}+="swupdate-usb@%k.service"