libbladeRF: Sped up flash writes, addressed log_info/debug consistency

Every flash page_write was being verified. Since we perform a readback
and verify, this is redundant and wastes time. This has been #ifdef'd
out (as it may come in handy for paranoid development checks from time
to time).

Some inconsistencies in log levels has been cleaned up.
libbladeRF: Support build with libusb-1.0.9

Although its use is not recommended (as later versions introduced USB
3.0-related fixes), the only thing holding back the build of
libusb-1.0.9 was a version check for debug purposes.

If libusb doesn't provide libusb_get_version() (1.0.10+), a dummy
function fills in a dummy string to denote this.
libbladeRF: Added debug/verbose log messages

These are intended to allow users to potentially glean a bit more info
in situations where the device is detected by the OS, but not by the
bladeRF library (and/or utils).
cmake: FX3 firmware build is disabled by default
libbladeRF: Continue probing for devices upon encountering failures

In cases where backends are unable to open a device, this allows another
backend to pick up the device. Specifically, there are a few controllers
that don't play nicely with libusb, but appear to work with the Cypress
driver/API.