qemu: export XEN_BOOT for make check

qemu-check.exp needs the environment variable $XEN_BOOT to be set,
or the following error is displayed:
 $ make check
 ...
 # no such variable
     (read trace on "::env(XEN_BOOT)")
     invoked from within
 "if {$::env(XEN_BOOT) == "y"} {
         info " (Xen Dom0)"
 }"
     (file "/<...>/qemu-check.exp" line 119)

Signed-off-by: Jerome Forissier <jerome@forissier.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>
Acked-by: Ruchika Gupta <ruchika.gupta@linaro.org>

qemu.mk

@@ -198,6 +198,7 @@ check: $(CHECK_DEPS)
 	cd $(BINARIES_PATH) && \
 		export QEMU=$(QEMU_BUILD)/arm-softmmu/qemu-system-arm && \
 		export QEMU_SMP=$(QEMU_SMP) && \
+		export XEN_BOOT=n && \
 		expect $(ROOT)/build/qemu-check.exp -- $(check-args) || \
 		(if [ "$(DUMP_LOGS_ON_ERROR)" ]; then \
 			echo "== $$PWD/serial0.log:"; \