Run eos-firstboot a second time on dual-boot systems

In new dual-boot installs of 6.0.0, eos-firstboot did not correctly grow
the filesystem due to the bug fixed in commit
56aea35.

We now want to run it a second time, only on dual-boot systems. Ideally
we would like to only trigger this on dual-boot systems that were
affected by this bug but it's harder to detect that case than to run
eos-firstboot again and have resize2fs gracefully do nothing.

When booting a non-live image-booted system which has not previously run
this unit, remove the flag file (if present) that prevents
eos-firstboot.service from running more than once, then touch another
flag file to prevent running this unit again. The service is ordered
Before=eos-firstboot.service and is Type=oneshot so we are guaranteed
that its ExecStart= line has completed before eos-firstboot.service's
ConditionPathExists check is evaluated. From systemd.unit(5):

> The conditions and asserts are checked at the time the queued start
> job is to be executed. The ordering dependencies are still respected,
> so other units are still pulled in and ordered as if this unit was
> successfully activated, and the conditions and asserts are executed
> the precise moment the unit would normally start and thus can validate
> system state after the units ordered before completed initialization.

On a fresh installation of an image containing this unit,
/var/eos-booted does not exist; so this unit has no effect but is
harmless.

On a system originally installed with 6.0.0, eos-firstboot.service will be
retriggered once and will successfully resize the filesystem to fill the
partition.

On a system originally installed with a release prior to 6.0.0,
eos-firstboot.service will be retriggered once and will successfully
take no action.

https://phabricator.endlessm.com/T35432

Makefile.am

@@ -25,6 +25,7 @@ dist_systemdunit_DATA = \
 	eos-live-boot-overlayfs-setup.service \
 	eos-ostree-bootloader-setup.service \
 	eos-prune-printers.service \
+	eos-secondboot.service \
 	eos-transient-setup.service \
 	eos-update-flatpak-repos.service \
 	eos-update-system-ca.service \

eos-secondboot.service

@@ -0,0 +1,19 @@
+# On dual-boot systems, (re)run eos-firstboot to grow the root filesystem, which
+# was not grown correctly on first boot into Endless OS 6.0.0.
+[Unit]
+Description=Fix dual-boot filesystem size
+DefaultDependencies=no
+After=sysinit.target local-fs.target
+Before=basic.target eos-firstboot.service
+ConditionPathExists=!/var/%N
+ConditionKernelCommandLine=endless.image.device
+ConditionKernelCommandLine=!endless.live_boot
+
+[Service]
+Type=oneshot
+ExecStart=rm -f /var/eos-booted
+ExecStartPost=touch /var/%N
+StandardOutput=journal+console
+
+[Install]
+WantedBy=basic.target