dhcpcd: Wait for carrier before active

In order to ensure that network interfaces are active, but not
necessarily fully configured, before *network.target* is reached, the
*dhcpcd.service* unit now waits until at least one interface has
processed the `CARRIER` event before being marked as active.  This is
accomplished using the *sd_notify* infrastructure, via the
`systemd-notify` command, executed in a client configuration hook
script.

package/dhcpcd/dhcpcd.mk

@@ -5,6 +5,9 @@ define DHCPCD_INSTALL_INIT_SYSTEMD
 	$(INSTALL) -D -m u=rw,go=r \
 		$(BR2_EXTERNAL_metricspi_PATH)/package/dhcpcd/dhcpcd-wait-online.service \
 		$(TARGET_DIR)/usr/lib/systemd/system/dhcpcd-wait-online.service
+	$(INSTALL) -D -m u=rwx,go=rx \
+		$(BR2_EXTERNAL_metricspi_PATH)/package/dhcpcd/ready.hook.sh \
+		$(TARGET_DIR)/usr/lib/dhcpcd/dhcpcd-hooks/01-ready
 	$(INSTALL) -D -m u=rwx,go=rx \
 		$(BR2_EXTERNAL_metricspi_PATH)/package/dhcpcd/online.hook.sh \
 		$(TARGET_DIR)/usr/lib/dhcpcd/dhcpcd-hooks/99-online

package/dhcpcd/dhcpcd.service

@@ -4,7 +4,8 @@ Wants=network.target
 Before=network.target
 
 [Service]
-ExecStart=/usr/sbin/dhcpcd -q --nobackground
+Type=notify
+ExecStart=/usr/sbin/dhcpcd -q --nobackground --env NOTIFY_SOCKET=${NOTIFY_SOCKET}
 
 [Install]
 WantedBy=multi-user.target

package/dhcpcd/ready.hook.sh

@@ -0,0 +1,5 @@
+#!/bin/sh
+
+if [ "${reason}" = CARRIER ]; then
+    systemd-notify --pid=${pid} --ready
+fi