Description of problem:
There is no facility for users to control the chunk depth of sections.
Version-Release number of selected component (if applicable):
Steps to Reproduce:
1. create_book -name foo
2. cd foo
3. make html
Sections chunked to depth of 5.
User should be able to override chunk depth.
Add Makefile param to allow users to overeride chunk.section.depth
Added CHUNK_SECTION_DEPTH param to allow chunk.section.depth override.