Staging: only build the tree if we really want to

This Kconfig change allows the common 'make allmodconfig' and
'make allyesconfig' build options to skip the staging tree, which is
probably what you want to have happen anyway.

This makes the linux-next developer's life a lot easier so he doesn't
have to worry about changes that break the staging tree, that's for me
to worry about...

Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>