LWIP_RFC7668_LINUX_WORKAROUND_PUBLIC_ADDRESS macro

LWIP_RFC7668_LINUX_WORKAROUND_PUBLIC_ADDRESS: Currently, the linux kernel driver for 6lowpan sets/clears a bit in the address, depending on the BD address (either public or not). Might not be RFC7668 conform, so you may select to do that (=1) or not (=0)