confd: bridge: Add STP/RSTP support

board/common/rootfs/etc/finit.d/available/mstpd.conf

@@ -0,0 +1,8 @@
+# A single mstpd instance can manage multiple bridges, which are
+# dynamically added/removed by the kernel via the /sbin/bridge-stp
+# usermode helper. We depend on usr/mstpd so that confd can
+# enable/disable the service without an initctl barrier, since it
+# needs to already be running when a bridge interface with spanning
+# tree enabled is created.
+service env:-/etc/default/mstpd <!usr/mstpd> \
+	 [S0123456789] mstpd $MSTPD_ARGS -- Spanning Tree daemon

board/common/rootfs/etc/finit.d/enabled/mstpd.conf

@@ -0,0 +1 @@
+../available/mstpd.conf