tests/docker: make test-mingw honour TARGET_LIST

The other builders honour this variable, so should the mingw build.
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>
Message-Id: <20161011161625.9070-4-alex.bennee@linaro.org>
Signed-off-by: NFam Zheng <famz@redhat.com>

tests/docker/test-mingw

@@ -16,9 +16,10 @@
 requires mingw dtc
 
 cd "$BUILD_DIR"
+DEF_TARGET_LIST="x86_64-softmmu,aarch64-softmmu"
 
 for prefix in x86_64-w64-mingw32- i686-w64-mingw32-; do
-    TARGET_LIST=x86_64-softmmu,aarch64-softmmu \
+    TARGET_LIST=${TARGET_LIST:-$DEF_TARGET_LIST} \
         build_qemu --cross-prefix=$prefix \
         --enable-trace-backends=simple \
         --enable-debug \