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/).