manual/user guide/customization: rename section 'Customizing packages'

This small patch renames the section 'Customizing packages' to 'Adding
project-specific patches'. Additionally, a minor change is done to the
introduction of this section.
To better reflect the contents, the source file is renamed.

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