[cloud] Retry DHCP aggressively in AWS EC2

[cloud] Retry DHCP aggressively in AWS EC2

The DHCP service in EC2 has been observed to occasionally stop
responding for bursts of several seconds.  This can easily result in a
failed boot, since the current cloud boot script will attempt DHCP
only once.

Work around this problem by retrying DHCP in a fairly tight cycle
within the cloud boot script, and falling back to a reboot after
several failed DHCP attempts.

Signed-off-by: Michael Brown <mcb30@ipxe.org>