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