libbladeRF: Fixed CMake check for libusb_get_version()

This change was missed in commit 34151f6e7.
libbladeRF: Added current working directory to file search path

The current working directory takes preference.
libbladeRF: Avoid truncating file names in file_find()

Skip attempts to check for the presence of a file when the
path and/or filename is truncated.
libbladeRF: Added support for BLADERF_SEARCH_DIR env var

If BLADERF_SEARCH_DIR environment variable is defined, it will
used by file_find() (e.g., for FPGA host-based autoloading) before
the other pre-defined paths (.e.g., ~/.config/Nuand/bladeRF/).
libbladeRF: Minor touch-up in handling file search of home dir

Ensure the path to the home directory isn't an empty string before
attempting to use it. Otherwise, this ends up just being a file
in the current working dir.
libbladeRF: Added support for finding files in dir of the running executable

This has been requested so that an FPGA image can be shipped alongside
a program that uses libbladeRF.