Description of problem:
When a book requires a notice about a particular trademark, we can presently:

* add the trademark to the Legal Notice shipped in the brand, and include it in every book, regardless of its relevance or otherwise
* include a custom legal notice for the book, which then must be manually maintained when the Legal Notice in the brand changes
* include the extra trademark information in some arbitrary part of the book

We could use a mechanism where extra trademark information could be included in the legal notice loaded from the brand. That way, we can keep the trademark information short, together, and relevant to the book in which it appears, while not sacrificing maintainability. 

Version-Release number of selected component (if applicable):
2.5

How reproducible:
100%

Steps to Reproduce:
1. Build a book
  
Actual results:
Cannot add to legal notice without overriding the whole file

Expected results:
Include extra trademark information without overriding the whole file

Additional info:
If additions to the legal notice were carried in a specially named XML file, it should probably be excluded from the PO mechanism; perhaps localized additions could be carried in the various language directories for anyone who specifically wants to provide translated legal notices.