manual/user guide/customization: refer to dir structure from rootfs section

This small patch adds references to the section on 'recommended directory
structure' from sections explaining the post-build script and rootfs
overlay.

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