map: add a legacymap option

map: add a legacymap option

The legacy map version based on the IPv6 Interface Identifier in
draft-ietf-softwire-map-03 was typically used by uncommenting the LEGACY
variable in the map.sh file, which is not ideal. A proper configuration
option is needed instead.

The IPv6 Interface Identifier format described in the draft was
eventually changed in RFC7597, but is still used by some major ISPs,
including in Japan.

Signed-off-by: Remi NGUYEN VAN <remi.nguyenvan+openwrt@gmail.com>
Signed-off-by: Hans Dedecker <dedeckeh@gmail.com> [PKG_RELEASE increase]