qemu, qemu_v8: add XTEST_ARGS

Implement a way to pass arguments to xtest when running "make check" or
"make check-only". For example:

 $ make check XTEST_ARGS="-x regression_1031"
 $ make check XTEST_ARGS="-t regression 1004 1013 1031"

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Acked-by: Etienne Carriere <etienne.carriere@linaro.org>
diff --git a/qemu_v8.mk b/qemu_v8.mk
index 6b41d3f..af85dea 100644
--- a/qemu_v8.mk
+++ b/qemu_v8.mk
@@ -456,6 +456,9 @@
 ifneq ($(CHECK_TESTS),)
 check-args += --tests $(CHECK_TESTS)
 endif
+ifneq ($(XTEST_ARGS),)
+check-args += --xtest-args "$(XTEST_ARGS)"
+endif
 
 check: $(CHECK_DEPS)
 	ln -sf $(ROOT)/out-br/images/rootfs.cpio.gz $(BINARIES_PATH)/