Add optionally-installed grub2 code

This pairs with coreos/bootupd#543
Basically we want bootupd to take care of installing this.  Add
the config here (where it more closely arguably belongs) so
that it can be optionally installed.

My proposal for at least the Fedora packaging of Ignition
is that we unconditionally depend on bootupd and install this,
because there's no one there using Ignition who doesn't want bootupd.

Makefile

@@ -30,6 +30,9 @@ install: all
 	ln -sf ../lib/dracut/modules.d/30ignition/ignition $(DESTDIR)/usr/libexec/ignition-apply
 	ln -sf ../lib/dracut/modules.d/30ignition/ignition $(DESTDIR)/usr/libexec/ignition-rmcfg
 
+install-grub-for-bootupd:
+	install -D -t $(DESTDIR)/usr/lib/bootupd/grub2-static/configs.d grub2/ignition.cfg
+
 .PHONY: vendor
 vendor:
 	@go mod vendor

grub2/ignition.cfg

@@ -0,0 +1,15 @@
+# Determine if this is a first boot and set the ${ignition_firstboot} variable
+# which is used in the kernel command line.
+set ignition_firstboot=""
+if [ -f "/ignition.firstboot" ]; then
+    # Default networking parameters to be used with ignition.
+    set ignition_network_kcmdline=''
+
+    # Source in the `ignition.firstboot` file which could override the
+    # above $ignition_network_kcmdline with static networking config.
+    # This override feature is also by coreos-installer to persist static
+    # networking config provided during install to the first boot of the machine.
+    source "/ignition.firstboot"
+
+    set ignition_firstboot="ignition.firstboot ${ignition_network_kcmdline}"
+fi