Makefile: expose target 'graph-build' to generate the build-time graphs

Generate the build-time graphs by calling:
    make graph-build

This generates the graphs in $(O)/graphs/

It is possible to use the alternate color-scheme by setting the variable
GRAPH_ALT=1 on the command line:
    make GRAPH_ALT=1 graph-build

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