libbladeRF: Check against VCO_LOW/VCO_HIGH.

When doing the search for the correct VCOCAP value, check to make
sure we are not equal to the edge stopping condition instead of
checking against VCO_NORM.  This avoids a problematic situation when
the found VCO_NORM value is on the cusp of being unlocked, causing
the upper bound to end prematurely.
libbladeRF: Added functions to check version numbers
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
bladeRF-cli: Bumped version to v0.10.8

Changes since v0.10.7 tag:
    - Updates to corresponding to libbladeRF API changes
    - Removed device reset after flashing firmware
    - Interactive help text command added to allow man page to
      be generated without needing a script to scrape text.
    - Preliminary support for XB-200
bladeRF-cli: Release "device control" lock while performing rx/tx wait

This fixes #251, which was a regression as the result of commit 85d1633.
By holding the "device control" lock while waiting, a completing rx/tx
stream operation would become deadlocked while attempting to disable its
associated module and notify the main task about the operation
completing.
WIP: Modify the RXVGA2 calibration routines.
Calibration update.
libbladeRF: Moved LPF DC tuning value write into address loop

This is just for readability. There's no functional change, since
there's only 1 address to loop over anyway.
libbladeRF: Added register settings from LMS FAQ to DC calibration fns

The following changes have been added to lms_dc_cal_loop() and
lms_calibrate_dc(), based upon recommendations from the LMS 6002D FAQ
(version 1.0r10).

    - Use the DC_REGVAL to determine success or failure, as DC_LOCK
      is reported (in the FAQ) to be unreliable. Retry if the result is
      31, setting the count to DC_CNTVAL to 0.

    - Enable DC comparators during DC calibration (default), and
      disable them upon completion to address linearity issues.

    - Disable DACs while performing TXLPF DC calibration
libbladeRF: Stubbed out autocal test program
Use linear interp to get first guess at DC offset values
Integer interpolation and apply rx auto cal parameters
Touched up output
Fixed bad interpolation, type promotion needed.
bladeRF-cli: Removed 'calibrate' cmd implementation

This command will be re-implemented using the autocalibration routines
added in following commits
libbladeRF: [WIP] Began adding RX DC autocal into lib
bladeRF-cli: 'calibrate' command performs RX DC calibration
libbladeRF: Preliminary bladerf_calibrate_dc for RX DC offset