Enhancement #512: B210: Need an Internal PPS
- Added support for internal PPS selection (set as default)
Fix for BUG #516: B210: Fails to Run with 30.72 MHz Clock
- Corrected clock rate checks for B2x0
Fix for BUG #516: B210: Fails to Run with 30.72 MHz Clock
    - Corrected clock rate checks for B2x0
Fix for BUG #492:  UHD: set_time_unknown_pps() fails with GPSDO installed
- Added polling for PPS time change after setting time from GPSDO.
x300: Bugfix for overflows on PCIe at 200MS/s
Fix for BUG #527: N200: 50 Msps results in two tones
- Adjusted check to enable first half-band filter only if the rate is decimated enough between the CIC and other half-band filter
BUG #516:  B210: Fails to Run with 30.72 MHz Clock
- Addressed feedback from review.
utils: uhd_usrp_probe gets argument --init-only

This will run the tool without the output (unless --tree is provided).
Using --init-only, uhd_usrp_probe can be used to simply test a device
in more detail than doing a uhd_find_device, but the output of the
device initalization is not scrolled by because of the device info
output.
Merge branch 'maint' into ashish/rx_pcie_overflows
Merge branch 'origin/n200/bug527' into maint

Fixing issue with 50 MSps resulting in two tones for N200.
Merge branch 'origin/b200/bug512' into maint

B200 now creates internal PPS. Depends on FPGA change.
Merge branch 'origin/ashish/rx_pcie_overflows' into maint
Merge remote-tracking branch 'origin/maint'
Merge branch 'origin/martin/utils/init_only_probe'
Merge branch 'origin/b200/bug516' into maint

Fixing B200 clock rate float compare.
Fix for BUG #469:  Bad/Empty GPS NMEA strings returned when the queries are made in a random wait iterative fashion
Fix for BUG #460:  X300: GPGGA sensor most often empty, while RMC is usually OK
- Added checksum verification of NMEA strings
- Improved handling of short or malformed strings
- Fixed GPSDO data synchronization between X300 firmware and host
Addressing comments from review.
- Corrected types of some variables to be boost types.
- Removed debugging code accidentally left in.
- Changed some compiled out error messages to log messages.
- Changed variables from uint8_t to uint32_t so parsing of hex strings would work properly.
Fix for BUG #469
- Added mutex for write_uart()
Updated copyright year.