Makefile: let show-targets prints also the dependencies

Because the "show-targets" target print the targets that will be built,
print also their dependencies.

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>