manual/developer guide: minor rework of 'adding board support' section

Minor rework of the section 'Creating your own board support', as follows:
- Rename section into 'Adding support for a particular board'
- Modify introduction
- Refer to Project-specific customizations chapter

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>