Restoring compatibility with CMake 2.6

* The UNSET command didn't exist back then, but using SET with no value does the same thing
* CMake 2.6 doesn't like nested parantheses in IF statements, so tweak IF/ELSE statements
cmake: fixed FindORC logic, was allowing usage of versions below minimum
- Fixes for channel alignment
	- Added analog delay for radio clock
	- Added analog delay for DAC reference clocks
	- Removed resetting of clock control
	- Removed setting of reference clock and PPS to external sources during initialization
- Fixes for set_time_unknown_pps
	- Removed wait for PPS edge after setting time from GPSDO
	- Changed set_time_unknonw_pps to time out based on system time rather than device VITA time
Changed analog delay on DAC reference and radio clocks from 1075ps to 900ps
Bumping FPGA compat to 7.
Updated CHANGELOG for 3.7.2rc2
fpga: Added FPGA code for X300 MIMO alignment bugfix
uhd: updated images url for UHD 3.7.2 release
fixup! uhd: updated images url for UHD 3.7.2 release
Merge branch 'maint' of https://github.com/EttusResearch/uhd

Conflicts:
	host/CMakeLists.txt