DragonBoard 820c support to CSI interface or not. if yes, then is there an example available that uses the MIPI CSI interface of DragonBoard 820c?. How to connect board to cloud?
You can use ov5640 or ov5645 camera with either D3 mezzanine  or Deltavision mezzanine .
A guide is available for 410c, but same apply for 820c:
Question is too vague, 820c has ethernet and wifi support.
As it is given it has OpenEmbedded OS support, so Does OpenEmbedded support CSI for 820c? If so which video compression it supports?
Is the D3 mezzanine is necessary if we use ov5640 or ov5645 for dragoanboard 820c?