Adds a buildPlugin function which can build Eagle Mode plugins that can
ultimately be linked into Eagle Mode using the existing etcDir function.
Change-Id: I338171779d3547faecbfb708fbaa78cd1cfd59ac
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12387
Autosubmit: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI
Reviewed-by: emery <emery@dmz.rs>
Reviewed-by: azahi <azat@bahawi.net>
