drivers: eth: native_posix: if_name from cmd line

Iface name which is used by native posix ethernet driver can only be
specified at compile-time. I wanted to run two instances of the same
program on native posix but did not want to make two separate builds only
to change the iface name. I have implemented getting the iface name from
command line.

Signed-off-by: Kacper Dalach <dalachowsky@gmail.com>