bladeRF-cli: Centralized checking of device presence and FPGA loaded state

These checks now occur once before a command is executed.

A subsequent commit will remove checking from the implementation of
each command.
bladeRF-cli: Removed per-command device presence and FPGA state checks

These checks are now performed in a single location prior to command
execution (in cmd.c), as of 2cf4111aed816748b91c06518a46d6d89ab16fe4.