Partial fix of hal/drivers/*.comp build

The `$(COMP_DRIVERS)` variable was left out of `obj-m`, so these comps
have not been built for a long time.

During that time, the pluto drivers seem to have broken completely.

The `pcl720` and `serport` comps appear not to have been built since
the advent of `USER_PCI`, and needed an untested header fix.

The `msgcomponents` comp recipes needed straightening out.

Fixes #384