initrd: add ip=... link6 mode for link-local only address assignment

this is useful for bringing up the interface with a IPv6 link local
address only during boot, e.g. to map a NBD device from a server that is
connected directly, e.g. via thunderbolt.
50 jobs for initrd-linklocal6 in 44 minutes and 33 seconds (queued for 6 seconds)