board: readme.txt for Intel Galileo Gen 1/2

Created a readme.txt for the Intel Galileo Gen 1/2 that describes,

 * How to build for the Intel Galileo Gen 1/2
 * How to access the console on the Intel Galileo Gen 1/2

Signed-off-by: Ray Kinsella <ray.kinsella@intel.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>