manual/user guide/customization: add section on layered customization

Inspired by some text in the 'project-specific patches' section, this patch
adds a separate section on layering customizations by providing multiple
post-build scripts, multiple rootfs overlays, etc.

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