Machine cleanup

As has been discussed a bit on the last couple of dev calls, there are     
some machine types that have combinations of the following issues:

• not buildable after the upgrade to Yocto Project 4.0 / kirkstone in master

• not obviously used by anyone in the community

• not easily tested by anyone active in the community

So to reduce our maintenance burden, I propose to remove the following

machine templates and their associated configuration (including BSP   
layer if applicable):

• hsdk

• dragonboard-410c

• dragonboard-820c

• nitrogen6x