core: remove redundant "override O := $(O)"

The top-level Makefile contains an "override O := $(O)" statement that
is purportedly required to make sure the O flag doesn't leak into the
environment of sub-makes. However, since commit 173135df, there is
already an "override O := ..." a few lines down. Therefore, the first
override is redundant.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>