dev: Build most of the networking devices in the NULL_ISA build.

dev: Build most of the networking devices in the NULL_ISA build.

The only part of these devices which are incompatible with other ISAs,
with the possible exception of endianness transformation, is that
the dist_iface implementation refers to ThreadContext methods and
that class is heavily tied to the guest ISA. Only those few lines are
excluded in a NULL_ISA build.

Change-Id: Ic6d643fdbb792d0a996a37d75e027c5ce0ecd460
Reviewed-on: https://gem5-review.googlesource.com/c/13469
Reviewed-by: Gabor Dozsa <gabor.dozsa@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>