Add QEMUEXTRA environment to add extra QEMU arguments, like in JOS

Makefile

@@ -201,7 +201,7 @@ QEMUGDB = $(shell if $(QEMU) -help | grep -q '^-gdb'; \
 ifndef CPUS
 CPUS := 2
 endif
-QEMUOPTS = -hdb fs.img xv6.img -smp $(CPUS) -m 512
+QEMUOPTS = -hdb fs.img xv6.img -smp $(CPUS) -m 512 $(QEMUEXTRA)
 
 qemu: fs.img xv6.img
 	$(QEMU) -serial mon:stdio $(QEMUOPTS)