package/Makefile.in: export O= to post-build/image scripts for out-of-tree builds

package/Makefile.in: export O= to post-build/image scripts for out-of-tree builds

Sometimes it can be interesting to call back into buildroot from a
post-build/image script (E.G. make printvars or similar). For this to work
correctly with out-of-tree builds we need to pass O= to make, but this is
currently not available in the environment of post-build/image scripts.

In concept, O could be derrived from BUILD_DIR (E.G. by stripping /build),
but directly exporting O is cleaner.

O= cannot be exported globally as it interferes with various build systems,
so instead add it to EXTRA_ENV.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>