networkd: extend dhcp timeout on failure

[maxux]

The DHCP request always tries for 20 seconds and then timeouts.
https://github.com/threefoldtech/zos/blob/main/pkg/network/dhcp/dhcp.go#L50-L51

On networks that have switches with spanning-tree enabled or anything related, these ports can take several seconds before starting to forward and 20 seconds after the port is brought up can be too short.

Just extending this timeout would increase boot time for everybody, which is not the best idea either. I think a good compromise would be testing with a small timeout first then increase it if it fails.

[xmonader]

Can this wait till the next release? or is it a blocker?

[maxux]

AFAIK this can wait, farmer affected fixed their switch.