Makefile: unexport O

After bee5745c ("Makefile: don't depend on the umask"), any use
of "make O=<dir>" would leak $O into the enviroment for submakes,
and it's inherited by package makefiles. Some package makefiles have
protections to make sure they don't use the value of $O if it comes from
the enviroment (Linux), but some don't (uClibc).

This caused build failures when using a different output dir.

Fix this by unconditionally unexporting the O variable, since we never
need to have it set in the environment for packages, it should be only
internally used by BR.

Signed-off-by: Guido Martínez <guido@vanguardiasur.com.ar>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>