[v2] Add a system programming option (pgm)

This patch adds an option to program your fpga from orpsoc.
Arguments specific to the backend programing tool can be
passed from orpsoc, for example:

      orpsoc pgm de1 --cable"USB-Blaster [2-1]"

Signed-off-by: Franck Jullien <franck.jullien@gmail.com>