[BSP] Update qemu script

bsp/qemu-vexpress-a9/qemu.bat

@@ -3,4 +3,4 @@ if exist sd.bin goto run
 qemu-img create -f raw sd.bin 64M
 
 :run
-qemu-system-arm -M vexpress-a9 -kernel rtthread.elf -serial vc -serial vc -sd sd.bin
+qemu-system-arm -M vexpress-a9 -kernel rtthread.elf -serial stdio -sd sd.bin

bsp/qemu-vexpress-a9/qemu.sh

-qemu-system-arm -M vexpress-a9 -kernel rtthread.elf -serial vc -serial vc
+if [ ! -f "sd.bin" ]; then
+dd if=/dev/zero of=sd.bin bs=64M count=1
+fi
+
+qemu-system-arm -M vexpress-a9 -kernel rtthread.elf -serial stdio -sd sd.bin