1. Change IP address format to addr/prefix, to be compatible with both IPv4/IPv6.
- When adding address in CIDR type, netmask/route will be automatically added.
2. Since route of whole subnet is added automatically, not specifying NuttX's IP any more.
- Multiple NuttX simulators (with IP 10.0.1.x) attached to same bridge can surf the net at same time.
3. NAT66 is used to make sure it works even if host has only one IPv6 address.
Signed-off-by: Zhe Weng <wengzhe@xiaomi.com>