Pass all the relevant sub-directory make variables.

Signed-off-by: Lluís Vilanova <vilanova@ac.upc.edu>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20140120112153.5685.30949.stgit@fimbulvetr.bsc.es