bladeRF-cli: Inform user if bootloader devices are present

Before attempting to open any devices when going into interactive mode,
running a script, or executing "-e" arguments, bladeRF-cli now checks
for any FX3-based devices in bootloader mode, and alerts the user
to their presence, recommending that he or she see 'help recover'
for more information.

Previously, a libbladeRF log_info() message was used to alert users
to the presence of FX3-based devices enumerating in bootloader mode.
This was removed to alleviate noise and because it was buried in
backend-specific code.

Given that the bladeRF-cli is intended to be the "go to" program for
debugging the state of a bladeRF device, this alert is more
appropriately located in this program.