This commit introduces the ability to connect a real hardware device to
the channel simulator.

It is possible to run an eNB with the libchannel_simulator.so as
liboai_device.so and the "usrp" program and have both connected
as if the eNB was directly talking to the USRP device.

For that, a channel can now have a "sample_advance". (The USRP receives
data from the air for subframe N and sends data for subframe N+x in the
future. In this commit x = 2 subframes.)

The following commands can be run to connect an eNB and the "usrp"
program in FDD mode with 25 RBs.

First, run the the channel simulator. In the directory channel_simulator:

    ./channel_simulator

To run usrp, as root, in the directory channel_simulator/usrp:

    ./usrp

To run the eNB, as root:

    CHANNEL_SIMULATOR_RX_FREQUENCY=2 CHANNEL_SIMULATOR_TX_FREQUENCY=1 CHANNEL_SIMULATOR_TX_SAMPLE_ADVANCE=15360 ./lte-softmodem -O [config file]

Then it is possible to connect a COTS UE.

It is also possible to connect openair UE which uses libchannel_simulator.so
as liboai_device.so. For that, run, as root:

    CHANNEL_SIMULATOR_RX_FREQUENCY=1 CHANNEL_SIMULATOR_TX_FREQUENCY=2 CHANNEL_SIMULATOR_RX_SAMPLE_ADVANCE=15360 ./lte-uesoftmodem -C 2680000000 -r 25 --ue-rxgain 140 --nokrnmod

Connecting both the openair UE and a COTS UE, it is possible to send traffic
between the two.

Note: performances need to be improved.

Note 2: the program "channel_simulator" now requires AVX2 instruction set.