Makefile: test if "dot" exists in "<pkg>-graph-depends"

To be able to check the "dot" command availability in
"<pkg>-graph-depends" move the check to the "graph-depends-requirements" rule.
Also don't use a subshell for the exit command to be sure that the error
will be returned by the shell.

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