Fix #367: add support for mdev's netlink daemon mode

This commit replaces the `mdev -s` call to populate the device tree with
the "new" `mdev -df` daemon mode, introduced in BusyBox 1.31.0, 2019.

In daemon mode mdev listens to kernel uevents, replacing the hotplug use
of mdev.  After creating the netlink socket, 'mdev -df' performs the same
initial scan as 'mdev -s' did.

To perform device (re)discovery, module loading and setup, including any
firmware loading, a coldplug operation is typically required.  This is
done by the new /libexec/finit/coldplug script, by Alexander Zangerl.

Unlike 'mdevadm settle', mdev does not offer any mechanism to detect when
the discovery operation is done: on slower systems the triggering side,
the coldplug script, often completes quite a bit earlier than mdev's
uevent processing.  I.e., depending on <run/coldplug/success> is not an
indicator of all devices having been (re)discovered and fully set up.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

configure.ac

@@ -15,7 +15,7 @@ AC_CONFIG_FILES([Makefile
 	man/Makefile
 	plugins/Makefile
 	src/Makefile
-	system/Makefile
+	system/Makefile system/10-hotplug.conf
 	tmpfiles.d/Makefile
 	test/test.env test/Makefile
 	test/lib/Makefile test/src/Makefile])