Is your feature request related to a problem? Please describe.
As a prerequisite to #554, the user needs to have the ability to specify their own set of extra modules to provide for the features they are customizing.

Describe the solution you'd like
A new setting in targets.cmake that allows users to add their own customization layers.

This is similar to but slightly different than loadable apps in that:

• The same set/config should be applied to all CPUs (i.e. if using for a message abstraction layer, all CPUs should share the same one). So it belongs as a global setting, rather than a setting associated with a single target.
• These would be linked with cfe core, rather than as dynamically-loaded libraries because something like a message abstraction layer is needed by SB itself, so it can't be loaded later.

Additional context
Initial function would be very simple - just include the libraries in the build, nothing more.

Requester Info
Joseph Hickey, Vantage Systems, Inc.