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>

dev: Fix race conditions at terminating dist-gem5 simulations

Two problems may arise when a distributed gem5 simulation terminates:
(i) simulation thread(s) may get stuck in an incomplete synchronisation
event which prohibits processing the simulation exit event; and (ii) a
stale receiver thread may try to access objects that have already been
deleted while exiting gem5. This patch terminates receive threads properly
and aborts the processing of any incomplete synchronisation event.

Change-Id: I72337aa12c7926cece00309640d478b61e55a429
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>

dev: Add m5 op to toggle synchronization for dist-gem5.
This patch adds the ability for an application to request dist-gem5 to begin/
end synchronization using an m5 op. When toggling on sync, all nodes agree
on the next sync point based on the maximum of all nodes' ticks. CPUs are
suspended until the sync point to avoid sending network messages until sync has
been enabled. Toggling off sync acts like a global execution barrier, where
all CPUs are disabled until every node reaches the toggle off point. This
avoids tricky situations such as one node hitting a toggle off followed by a
toggle on before the other nodes hit the first toggle off.

dev: Add 'simLength' parameter in EthPacketData
Currently, all the network devices create a 16K buffer for the 'data' field
in EthPacketData, and use 'length' to keep track of the size of the packet
in the buffer. This patch introduces the 'simLength' parameter to
EthPacketData, which is used to hold the effective length of the packet used
for all timing calulations in the simulator. Serialization is performed using
only the useful data in the packet ('length') and not necessarily the entire
original buffer.

dev: Exit correctly in dist-gem5
The receiver thread in dist_iface is allowed to directly exit the simulation.
This can cause exit to be called twice if the main thread simultaneously wants
to exit the simulation. Therefore, have the receiver thread enqueue a request
to exit on the primary event queue for the main simulation thread to handle.

style: eliminate explicit boolean comparisons

Result of running 'hg m5style --skip-all --fix-control -a' to get
rid of '== true' comparisons, plus trivial manual edits to get
rid of '== false'/'== False' comparisons.

Left a couple of explicit comparisons in where they didn't seem
unreasonable:
invalid boolean comparison in src/arch/mips/interrupts.cc:155
>> DPRINTF(Interrupt, "Interrupts OnCpuTimerINterrupt(tc) == true\n");<<
invalid boolean comparison in src/unittest/unittest.hh:110
>> "EXPECT_FALSE(" #expr ")", (expr) == false)<<

dev: Distributed Ethernet link for distributed gem5 simulations

Distributed gem5 (abbreviated dist-gem5) is the result of the
convergence effort between multi-gem5 and pd-gem5 (from Univ. of
Wisconsin). It relies on the base multi-gem5 infrastructure for packet
forwarding, synchronisation and checkpointing but combines those with
the elaborated network switch model from pd-gem5.