core: split variables definition related to in/out-of-tree build from O itself

This change uncorrolates the CONFIG_DIR and NEED_WRAPPER definition from
the presence of the O variable in the command line.

Now, the condition used to set these variables is the value of O itself.

This change is a preparatory work since the O definition will need to
be moved around when we will make Buildroot run with absolute canonical
paths for both its root directory and the output location.
This will be addressed in a follow-up patch.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Tested-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>