Support balena fin (CM3)

[agherzan]

The new device type will have the slug fincm3 and will specify a subtype_of being raspberrypi3.

This was discussed in the architecture call (2018-05-01). See: https://docs.google.com/document/d/1TiA3kt7zU8ce9mlNl5EWymfQ41VWW_xPZGHSMgDORPw/edit

[floion]

When was this discussed? @shaunmulligan I thought there would not be a new device type for the fin (as per a discussion only a few days ago)

[shaunmulligan]

@floion yeah it was discussed in arch call, we are gonna make a new device type that is a subtype of the rpi

[floion]

Then I guess we can do this for other Balena Fin like boards? Or this is just a one-timer?