We’re not using the arch, os and cpu options in build target config. These were
meant for cross compilation and we may not even use them in the future, so
remove them. 

Remove the use_first_core option in execution config and modify the logic as if
use_first_core=True if the lcore option is not set. Also  properly document
this (in code and docs) and also emit an info log when skipping the first core
and a warning when using the core (something like only do this if you know what
you’re doing).

There are a lot of supported devices in the schema, trim down.

We may also remove the OS/Arch config of SUT nodes, as we should be able to
discover that.

We may want to do additional config validation, as the schema doesn't cover
everything.