build: remove extra-obj-y

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: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
