Configuration: Differentiate between loopback traffic and loopback device

Configuration: Differentiate between loopback traffic and loopback device

This commit enables configuring handling of loopback traffic
separately from loopback device. Loopback traffic allows to
send packets with a destination IP address that is equal to
the netif device IP address, and looping them back up the
stack.
The loopback interface provides an interface for localhost
communication (127.0.0.1). The loopback interface
configuration is moved under the `Netif drivers` submenu.

Signed-off-by: Simon Kuenzer <simon.kuenzer@neclab.eu>
Reviewed-by: Sergiu Moga <sergiu.moga@protonmail.com>
Approved-by: Razvan Deaconescu <razvan.deaconescu@cs.pub.ro>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Pull-Request: #11