phylib does not wait for link

[Sascha Hauer]

The current phylib behaviour is unusable for unattended network boot, we
need to do something about it.

We currently have no instance that would wait until a link comes up.
This may work for the default-3-seconds-boot-timeout which is enough for
most phys, but fails with smaller timeouts.

There are several places in which we could wait for a link to come up, I
have no idea which one is the best:

- eth_send
- phylib
- in the protocols (dhcp, ping, ...)

Anyway, this deserves fixing before the next release.

Sascha

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