Allow grace period when receiving a fast OOB ack

A boot server can take more than 250ms to reply, especially to a pairing
request. When the boot server replies with a fast OOB ack (CTL and ACK bits
on, zero payload length), jeeboot is informed that its request is being
processed. It can therefore introduce a grace period by extending the 250ms
timeout to 2.5secs. If no such fast OOB ack is received, the original timeout
logic is kept, maintaining backwards compatibility.