UDP application sends packet using esp_netif, underlying transport such
as Wi-Fi may drop the packet due to higher load. New error code
represent transient, non-fatal packet drop error. udp application may
use such errtype, for example to rate limit.
1. Move adding of prefix/postfix to the private key to example code.
This is to preserve forward compatibility when more curves are supported.
2. Documentation correction of bootstrapping key configuration.
