extra-obj-y is somewhat complicated to understand.  Replace it with a
special CONFIG_ALL symbol that is defined only at toplevel.
This limits the case of directories defining more than one
*-obj-y target.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>