Retry when DHCP fails

Previously our bootstrap script would simply fail if DHCP didn't work the
first time around; this extends it to retry up to 60 times with an increasing
delay before finally giving up and doing a full reboot.

This should help out users with slow networks, which is often caused by
switches that take a little time to enable forwarding, bridging, network layer
authentication, or other related tools frequently enabled on "enterprise"
quality networks.

Signed-off-by: Daniel Pittman <daniel@rimspace.net>