Fix for BUG #500:  B210: RX channels are not phase aligned
- Adding UHD side code to invert second RX channel
- Changed XOR to OR for REG_DSP_RX_MUX flags.
Merge branch 'origin/b200/bug500' into maint - Fixing B200 phase alignment issue
Fixing help text for --ref argument in some examples. Reported by Marcus
Mueller.