Makefile: remove the graphs/ dir on 'make clean'

Currently, a 'make clean' leaves the graphs/ subdirectory in the
output directory. This commit defines a GRAPHS_DIR variable, used by
the different graph-generating targets, and which gets cleaned up in
the 'clean' target.

[Thomas: use the new GRAPHS_DIR variable in more places, as suggested
by Yann.]

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>