dev: add support for multi gem5 runs

Multi gem5 is an extension to gem5 to enable parallel simulation of a
distributed system (e.g. simulation of a pool of machines
connected by Ethernet links). A multi gem5 run consists of seperate gem5
processes running in parallel (potentially on different hosts/slots on
a cluster). Each gem5 process executes the simulation of a component of the
simulated distributed system (e.g. a multi-core board with an Ethernet NIC).

The patch implements the "distributed" Ethernet link device
(dev/src/multi_etherlink.[hh.cc]). This device will send/receive
(simulated) Ethernet packets to/from peer gem5 processes. The interface
to talk to the peer gem5 processes is defined in dev/src/multi_iface.hh and
in tcp_iface.hh.

There is also a central message server process (util/multi/tcp_server.[hh,cc])
which acts like an Ethernet switch and transfers messages among the gem5 peers.

A multi gem5 simulations can be kicked off by the util/multi/gem5-multi.sh
wrapper script.

Checkpoints are supported by multi-gem5. The checkpoint must be
initiated by a single gem5 process. E.g., the gem5 process with rank 0
can take a checkpoint from the bootscript just before it invokes
'mpirun' to launch an MPI test. The message server process will notify
all the other peer gem5 processes and make them take a checkpoint, too
(after completing a global synchronisation to ensure that there are no
inflight messages among gem5).

src/dev/SConscript

@@ -60,6 +60,10 @@ Source('etherdevice.cc')
 Source('etherdump.cc')
 Source('etherint.cc')
 Source('etherlink.cc')
+Source('multi_packet.cc')
+Source('multi_iface.cc')
+Source('multi_etherlink.cc')
+Source('tcp_iface.cc')
 Source('etherpkt.cc')
 Source('ethertap.cc')
 Source('i2cbus.cc')
@@ -85,6 +89,8 @@ DebugFlag('DiskImageWrite')
 DebugFlag('DMA')
 DebugFlag('DMACopyEngine')
 DebugFlag('Ethernet')
+DebugFlag('MultiEthernet')
+DebugFlag('MultiEthernetPkt')
 DebugFlag('EthernetCksum')
 DebugFlag('EthernetDMA')
 DebugFlag('EthernetData')