cli: Added FX3/ FPGA loading support and filled in misc. cmd stubs.

The CLI now supports flashing the FX3 and loading the FPGA from the
command line and through the interactive console.

To flash the firmware from the command line:
  ./bin/bladerf -b -f ./path/to/img -d /dev/bladerf1

To load the firmware from the command line:
  ./bin/bladerf -b -l ./path/to/rbf -d /dev/bladerf1

To flash/load interactively either run with -d /dev/bladerf1, or
use the open command to attach to a device:
  bladeRF> open /dev/bladerf1

To flash the firmware interactively:
  bladeRF> load fx3 ./path/to/img

To load the FPGA interactively:
  bladeRF> load fpga ./path/to/rbf

Note that a power-on-reset is currently required in between
flashing firmware and loading the FPGA.

Various commands have been stubbed out for the time being. A TODO
list has been created to keep track of desired functionality.

The simple_load and upgrade_firmware programs have been removed,
as they are no longer needed.