Merge remote-tracking branch 'remotes/stefanha/tags/tracing-pull-request' into staging

Pull request

Tracing makefile fixes for QEMU 2.9.

# gpg: Signature made Thu 16 Mar 2017 06:56:10 GMT
# gpg:                using RSA key 0x9CA4ABB381AB73C8
# gpg: Good signature from "Stefan Hajnoczi <stefanha@redhat.com>"
# gpg:                 aka "Stefan Hajnoczi <stefanha@gmail.com>"
# Primary key fingerprint: 8695 A8BF D3F9 7CDA AC35  775A 9CA4 ABB3 81AB 73C8

* remotes/stefanha/tags/tracing-pull-request:
  trace: ensure $(tracetool-y) is defined in top level makefile
  makefile: generate trace-events-all upfront
  makefile: merge GENERATED_HEADERS & GENERATED_SOURCES variables

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>