build: Fix dtc-checkout race condition in Makefile

This was introduced by:
    commit aef45d51d1204f3335fb99de6658e0c5612c2b67
    Author: Daniel P. Berrange <berrange@redhat.com>
    Date:   Fri Sep 29 11:11:56 2017 +0100

	build: automatically handle GIT submodule checkout for dtc

On my system, I see the following with a fresh clone:

% ./configure --disable-gtk --target-list=aarch64-softmmu
% make -j8
  GEN     aarch64-softmmu/config-devices.mak.tmp
  GEN     config-host.h
mkdir -p dtc/libfdt
  GIT     ui/keycodemapdb dtc
mkdir -p dtc/tests
  GEN     qemu-options.def
[snip]
  GEN     migration/trace.h
make: *** [git-submodule-update] Error 1
make: *** Waiting for unfinished jobs....

Upon closer inspection, the root cause of the error is:

% git submodule update --init ui/keycodemapdb dtc
fatal: destination path 'dtc' already exists and is not an empty directory.
Clone of 'git://git.qemu-project.org/dtc.git' into submodule path 'dtc' failed

This patch fixes this race condition by forcing the 'dtc/%' rule which caused
'dtc' to be non-empty to wait on '.git-submodule-status'.

Signed-off-by: Aaron Lindsay <alindsay@codeaurora.org>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Acked-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 1508352023-28591-1-git-send-email-alindsay@codeaurora.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
