Add a CI test that builds a CAmkES system with a DTB connector

[axel-h]

Having a CI test that builds a CAmkES system with a DTB connector would have caught the missing thing from 7e47639 that
#28 adds.

With QEMU/sabre we can build a UART or Network enabled CAmkES system. Building should be enough to have caught the error, running this to test if the device works might require more work then.