Makefile: out-of-tree fix for uclibc 0.9.31

Strangely enough O is still passed to submakes with MAKEOVERRIDES
(with make 3.81 atleast), the only thing that changes is the output
of the origin function (command line -> environment).
Unfortunately some packages don't look at origin (E.G. uClibc 0.9.31+)
To really make O go away, we have to override it.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>