manual/user guide/customization: rework section on storing configuration

This patch promotes the sections on Buildroot configuration and the
configuration of other components to top-level sections, to match the
overview in the introduction of the chapter. The introduction of the
original section is removed as it does not bring additional info anymore.

Some rewording is done as well.

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