dhcpcd: Fix startup process

The *dhcpcd.service* systemd unit file that ships with Buildroot is
broken.  It orders the unit after *network.target*, which makes no
sense.  It also configures the process to fork, but incorrectly
configures the PID file.  Finally, by relying on the fork to indicate
that the network is "online," it does not distinguish between "an
interface is up" and "the network is reachable."  As most daemons only
need the former, they only rely on *network.target*, and thus can fail
to start correctly.

To correct this brokenness, we provide our own unit file for
*dhcpcd.service*, based on the one included in the Fedora *dhcpcd* RPM
package.  For "online" signalling, we provide a pair of shell scripts:
one reads from a named pipe waiting for a message and the other sends a
message to the pipe when configuration is complete.

package/dhcpcd/dhcpcd-wait-online.service

@@ -0,0 +1,16 @@
+[Unit]
+Description=Wait for Network to be Configured
+DefaultDependencies=no
+Conflicts=shutdown.target
+After=dhcpcd.service
+Requires=dhcpcd.service
+Before=network-online.target shutdown.target
+
+[Service]
+Type=oneshot
+ExecStart=/usr/libexec/dhcpcd-wait-online
+TimeoutStartSec=1m
+RemainAfterExit=yes
+
+[Install]
+WantedBy=network-online.target