graph-depends: fix when $(O) is a relative path

The graph-depends commands cd into the CONFIG_DIR and run the script
from there. However, this means that when $(O) is a relative path, it
will no longer be correct.  Therefore, use $(BASE_DIR) instead of $(O).

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
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>