docs/manual: slightly reword the solutions to customize rootfs

The order of the solutions to customize the root filesystem is
changed: we now mention the post-build script mechanism *before* the
custom root filesystem skeleton mechanism, because the former is
preferred over the latter.

In addition to this, we give a few more details about direct
customization of the root filesystem in output/target, and about the
custom target skeleton solution.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: Luca Ceresoli <luca@lucaceresoli.net>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>