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>

src/Makefile.am

@@ -19,6 +19,8 @@ if WATCHDOGD
 pkglibexec_PROGRAMS += watchdogd
 endif
 
+dist_pkglibexec_SCRIPTS = coldplug
+
 finit_pkglibdir      = $(pkglibdir)
 finit_pkglib_SCRIPTS = rescue.conf sample.conf