debian: add packages to download FPGA/FX3 imgs

Based somewhat loosely on prism2-usb-firmware-installer.
Add /usr/share/bladerf/fpga/ to search path

Will allow bitstreams downloaded by the debian pkgs to be auto-discovered!
Change FPGA/Firmware download path to be cleaner

Now using /usr/share/Nuand/bladeRF for everything, instead of
/usr/share/bladerf/{firmware,fpga}
Merge pull request #276 from rtucker/dev-issue166

Add Debian packages to download FPGA/FX3 images (Fixes #166)