Add gokrazy.WaitForNetwork()

[stapelberg]

Currently, we use gokrazy.WaitForClock() as a catch-all for clock and network.

This works well on the Raspberry Pi, which doesn’t have an RTC chip on board, and hence needs the network before the clock can be set.

But, in qemu (and other devices that have a functioning clock on every boot), this doesn’t work.

We should add some sort of connectivity checking.

[oliverpool]

My 2 cents: call it gokrazy.WaitForClockAndNetwork, so that it can be used for https requests (if the clock is wrong, the certificate verification should fail).