support/scripts: do not display virtual packages in generated lists

If a package has both a 'real' and a 'virtual' definition, consider it
is a virtual package and do not display it in the generated package list.

This is the case for jpeg and cryptodev, that are virtual packages, but
also real (but empty) packages used to provide a prompt to enable/disable
a choice to select an implementation. In this case, we do not want to
list the virtual packages, but only their implementations.

So, consider packages that are both real and virtual as virtual packages.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Samuel Martin <s.martin49@gmail.com>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>