Add extra_nic option

kernel/Makefile

@@ -27,6 +27,7 @@
 #         | raspi3              Only available on aarch64, run on Raspberry Pi 3 Model B/B+
 #   pci_passthru = 0000:00:00.1 Only available on x86_64, passthrough the specified PCI device
 #   init = /bin/ls              Only available on riscv64, run specified program instead of user shell
+#   extra_nic = on | off         Only available on x86_64, add an additional e1000 nic
 
 arch ?= riscv64
 board ?= none
@@ -36,6 +37,7 @@ graphic ?= off
 smp  ?= 4
 pci_passthru ?=
 init ?=
+extra_nic ?= off
 
 target := $(arch)
 build_path := target/$(target)/$(mode)
@@ -81,6 +83,11 @@ qemu_opts += \
 qemu_net_opts += \
 	-device vfio-pci,host=$(pci_passthru)
 endif
+ifeq ($(extra_nic), on)
+qemu_net_opts += \
+	-netdev type=tap,id=net1,script=no,downscript=no \
+	-device e1000e,netdev=net1
+endif
 
 else ifeq ($(arch), riscv32)
 qemu_opts += \