Update FCOS iPXE initrd and kernel arg settings

* Add initrd=main kernel argument for UEFI
* Switch to using the coreos.live.rootfs_url kernel argument
instead of passing the rootfs as an appended initrd
* Remove coreos.inst.image_url kernel argument since coreos-installer
now defaults to installing from the embedded live system
* Remove rd.neednet=1 and dhcp=ip kernel args that aren't needed
* Remove serial console kernel args by default (these can be
added via var.kernel_args if needed)

Rel:
* poseidon/matchbox#972 (thank you @bgilbert)
* poseidon/matchbox#978

profiles.tf

@@ -18,8 +18,6 @@ resource "matchbox_profile" "flatcar-install" {
     "initrd=flatcar_production_pxe_image.cpio.gz",
     "flatcar.config.url=${var.matchbox_http_endpoint}/ignition?uuid=$${uuid}&mac=$${mac:hexhyp}",
     "flatcar.first_boot=yes",
-    "console=tty0",
-    "console=ttyS0",
     var.kernel_args,
   ])
 
@@ -42,8 +40,6 @@ resource "matchbox_profile" "cached-flatcar-install" {
     "initrd=flatcar_production_pxe_image.cpio.gz",
     "flatcar.config.url=${var.matchbox_http_endpoint}/ignition?uuid=$${uuid}&mac=$${mac:hexhyp}",
     "flatcar.first_boot=yes",
-    "console=tty0",
-    "console=ttyS0",
     var.kernel_args,
   ])