base-files: send informational UDP message each second waiting

base-files: send informational UDP message each second waiting

The preinit network initialisation and failsafe informational message
are inherently racy as the interface takes some time to become
functional after "ip link set $pi_ifname up" command.

Consider this timing:

[   12.002713] IPv6: ADDRCONF(NETDEV_UP): eth1: link is not ready
[   12.008819] IPv6: ADDRCONF(NETDEV_UP): eth1.1: link is not ready
[   12.118877] random: procd: uninitialized urandom read (4 bytes read)
[   13.068614] eth1: link up (1000Mbps/Full duplex)
[   13.073309] IPv6: ADDRCONF(NETDEV_CHANGE): eth1: link becomes ready
[   13.080445] IPv6: ADDRCONF(NETDEV_CHANGE): eth1.1: link becomes ready

Since the UDP message was sent prior to link becoming ready, it was
never seen on the wire.

The default failsafe timeout is set to 2 seconds, so with this patch
there are two attempts to send the message, one spent in vain, and the
other visible in tcpdump on an attached host. Of course, in cases when
the interface is brought up faster it leads to two messages, however it
should be harmless. This patch (almost) doesn't affect normal boot time
while still allowing to enter failsafe reliably with a single button
press, matching the official "generic failsafe" documentation.

Signed-off-by: Paul Fertser <fercerpav@gmail.com>