Automatically postpone automated reboots by deferable-reboot

[ilario]

As explained in #1023, deferable-reboot was developed in two pieces, the main one on this repository and a very useful piece in the LibreRouterOS repository.

Without the second piece, the routers would just reboot every 27 hours, annoying.
With the second piece, the routers are checking the internet connectivity every 3 minutes (by means of a ping to a known IP on the internet) and the reboot is deferred by 13 minutes if the ping is successful.

With this PR, the second piece is also included in the deferable-reboot package in this repository.

The IP to be pinged has been de-hardcoded and moved to the LibreMesh configuration files in the system section.

The chosen IP is one of the ones being used as DNS:

lime-packages/packages/lime-docs/files/www/docs/lime-example.txt

Lines 48 to 49 in 6642f37

	list resolvers 4.2.2.2 # b.resolvers.Level3.net # DNS servers node will use
	list resolvers 141.1.1.1 # cns1.cw.net # Set every entry empty for using the upstream (ISP) DNS server

I am testing in these days, please do not merge yet.

[ilario]

Tested and it works.
@germanferrero once this gets merged, you should consider removing the file in the LibreRouterOS repository https://gitlab.com/librerouter/librerouteros/-/blob/librerouter-1.5/files/etc/uci-defaults/90-cron-defer-reboot

[ilario]

Just increased a bit the deferring time (from 13 min to 30 min) in order to be sure that the network had time to re-establish the connection after a failure.
Also, added a random delay in the execution of the ping for not bombarding too hard the 4.2.2.2 with pings (does it ban IPs pinging too much?).