package infra: remove duplicates in dependencies list

Currently, we just use what a package declares as its dependencies.

But some packages may declare the same depdency more than once. For
example, php has two options to add SQL support: 'mysql' or 'mysqli',
which are not exclusive. So, php.mk has mysql twice as a dependency.

Although that does not cause any grievance for make, we end up generating
dependency graphs where this duplicate dependency is visible.

Add an intermediary variable which contains the $(sort)-ed list of the
dependencies, thus eliminating any duplicates.

This has the side effect of also sorting the list, which is probably
good for reproducibility anyway.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>