Add support for Raspberry PI3

[aabadie]

This PR adds support in Yocto for the Raspberry PI3 board.

For the moment, it's possible to use it as a gateway but with no control node. For this gateway setup, basic features has been tested: start/stop an experiment, flash a board, serial redirection.

How to build images and package

In this PR, the base Makefile has been modified to easily manage the builds of A8 and RPI3 boards (or targets). I just had to make the choice of renaming the target images - iotlab-image-open-a8 and iotlab-image-open-a8-autotest - to more generic names : iotlab-image and iotlab-image-autotest. iotlab-image-gateway is kept unchanged.

To summarize, building things using make can be done like this:

# Build the gateway image for a8:
make iotlab-image-gateway
# Build the open-a8 image:
make iotlab-image
# Build the open-a8 autotest image (not yet available for RPI3):
make iotlab-image-autotest
# Build the gateway image for Raspberry PI3:
make TARGET=rpi3 iotlab-image-gateway
# Build the open linux image for Raspberry PI3:
make TARGET=rpi3 iotlab-image

A8 target has extra builds available: uboot and build-kernel-mtd-rw

For each target, one can build all targets with following calls:

# For A8 (this hasn't changed):
make build-all
# For Raspberry PI3:
make TARGET=rpi3 build-all

With recent changes in install-lib, the use of Raspberry PI3 as a gateway is now working (without control nodes: no monitoring available).

[aabadie]

I have an ACK from @fsaintma, so let's go and have basic Raspberry PI 3 support in IoT-LAB !

[aabadie]

Now that this is merged, I updated this wiki page : https://github.com/iot-lab/iot-lab/wiki/Yocto-Build-System.