qemu: use 1 virtual CPUs (-smp 1) for "make check"

It seems that Shippable CI does not like running QEMU with 4 virtual
cores (or maybe we have a bug), because "make check" hangs when QEMU
is started with -smp 4. Revert to 1 core by default.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Joakim Bech <joakim.bech@linaro.org>

qemu.mk

@@ -188,7 +188,7 @@ ifneq ($(TIMEOUT),)
 check-args += --timeout $(TIMEOUT)
 endif
 
-QEMU_SMP ?= 4
+QEMU_SMP ?= 1
 check: $(CHECK_DEPS)
 	cd $(BINARIES_PATH) && \
 		export QEMU=$(ROOT)/qemu/arm-softmmu/qemu-system-arm && \