Add GRUB firstboot variable support

For firstboot detection Ignition currently supports the flag file
/boot/ignition.firstboot. On a pure read only root file system however this
file could not be deleted, so there has to be another option for storing
the flag.

This commit implements support for storing the flag as a GRUB environment
variable.

grub/02_ignition_firstboot

@@ -7,7 +7,6 @@ exec tail -n +3 $0
 search --set=bootpart --label boot
 # Determine if this is a first boot and set the variable
 # to be used later on the kernel command line.
-set ignition_firstboot=""
 if [ -f "(${bootpart})/ignition.firstboot" ]; then
     set ignition_firstboot="ignition.firstboot rd.neednet=1 ip=dhcp"
 fi

systemd/ignition-firstboot-complete.service

@@ -17,7 +17,11 @@ RemainAfterExit=yes
 # detected this file. Fail if we are unable to remove it, rather than risking
 # rerunning Ignition at next boot.
 MountFlags=slave
-ExecStart=/bin/sh -c 'mount -o remount,rw /boot && rm /boot/ignition.firstboot'
+ExecStart=/bin/sh -c 'if [ -e /boot/ignition.firstboot ]; then \
+    mount -o remount,rw /boot && rm /boot/ignition.firstboot; fi'
+ExecStart=-/bin/sh -c 'if command -v grub2-editenv >/dev/null && \
+    grub2-editenv list | grep -q "^ignition_firstboot="; then \
+    grub2-editenv - set ignition_firstboot=0; fi'
 
 [Install]
 WantedBy=multi-user.target