fx3: Bumped version to 1.7.0

Version bumped to denote added firmware loopback feature.
libbladeRF: Check for required FW version when enabling FW loopback
libbladeRF: Bumped revision to v0.15.0

Changes since v0.14.0 tag include:
 - Overhauled backend interface to decouple device logic from libusb
   code. Moved code to a generic USB interface and libusb code under
   that interface. This should allow host-specific drivers to be more
   easily integrated.
 - Addressed race conditions in the async RX/TX interface. pthreads is
   now a required dependency of libbladeRF.
 - Bug fixes for the sync RX/TX interface
 - Windows build and compatibility fixes
 - API changes:
    + Added functions:
        - FPGA timestamp access (requires FPGA v0.0.4+)
        - Expansion board GPIO and SPI accessors (requires FPGA v0.0.4+)
        - Firmware loopback (requires FX3 FW v1.7.0+)
    + Changed SPI flash access functions
        - Functions operate in units of erase blocks and pages
        - Removed un-aligned accesses