network: Allow to configure addr_gen_mode

Defines how link-local and autoconf addresses are generated.

	0: generate address based on EUI64 (default)
	1: do no generate a link-local address, use EUI64 for addresses generated
	   from autoconf
	2: generate stable privacy addresses, using the secret from
	   stable_secret (RFC7217)
	3: generate stable privacy addresses, using a random secret if unset