Makefile: add variable print capabilities

Add a new goal that dumps all of our variables.

Taken from http://www.cmcrossroads.com/ask-mr-make/6521-dumping-every-makefile-variable



Signed-off-by: Emeric Vigier <emeric.vigier@savoirfairelinux.com>
[yann.morin.1998@free.fr: drop per-variable print; refresh patch]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>