Makefile: enable using eth1 as tap device

e.g. run `sudo make TAP=tap0 run` to not autoconfigure
the second interface and just let it run as a tap device.

You then have to bring your own dhcp server and setup
everything manually.

Signed-off-by: Christoph Ostarek <christoph@zededa.com>

Makefile

@@ -249,6 +249,11 @@ QEMU_TPM_DEVICE_riscv64=tpm-tis
 QEMU_OPTS_TPM_Y_$(ZARCH)=-chardev socket,id=chrtpm,path=$(CURRENT_SWTPM)/swtpm-sock -tpmdev emulator,id=tpm0,chardev=chrtpm -device $(QEMU_TPM_DEVICE_$(ZARCH)),tpmdev=tpm0
 QEMU_OPTS_TPM=$(QEMU_OPTS_TPM_$(TPM:%=Y)_$(ZARCH))
 
+QEMU_OPTS_eth1=-netdev user,id=eth1,net=$(QEMU_OPTS_NET2),dhcpstart=$(QEMU_OPTS_NET2_FIRST_IP) -device virtio-net-pci,netdev=eth1,romfile=""
+ifneq ($(TAP),)
+QEMU_OPTS_eth1=-netdev tap,id=eth1,ifname=$(TAP),script="" -device virtio-net-pci,netdev=eth1,romfile=""
+endif
+
 QEMU_OPTS_amd64=-smbios type=1,serial=$(QEMU_EVE_SERIAL)
 QEMU_OPTS_arm64=-smbios type=1,serial=$(QEMU_EVE_SERIAL) -drive file=fat:rw:$(dir $(DEVICETREE_DTB)),label=QEMU_DTB,format=vvfat
 QEMU_OPTS_riscv64=-kernel $(UBOOT_IMG)/u-boot.bin -device virtio-blk,drive=uefi-disk
@@ -259,7 +264,7 @@ QEMU_OPTS_COMMON= -m $(QEMU_MEMORY) -smp 4  $(QEMU_OPTS_BIOS) \
         -global ICH9-LPC.noreboot=false -watchdog-action reset \
         -rtc base=utc,clock=rt \
         -netdev user,id=eth0,net=$(QEMU_OPTS_NET1),dhcpstart=$(QEMU_OPTS_NET1_FIRST_IP),hostfwd=tcp::$(SSH_PORT)-:22$(QEMU_TFTP_OPTS) -device virtio-net-pci,netdev=eth0,romfile="" \
-        -netdev user,id=eth1,net=$(QEMU_OPTS_NET2),dhcpstart=$(QEMU_OPTS_NET2_FIRST_IP) -device virtio-net-pci,netdev=eth1,romfile="" \
+	$(QEMU_OPTS_eth1) \
         -device nec-usb-xhci,id=xhci \
         -qmp unix:$(CURDIR)/qmp.sock,server,wait=off
 QEMU_OPTS_CONF_PART=$(shell [ -d "$(CONF_PART)" ] && echo '-drive file=fat:rw:$(CONF_PART),format=raw')