migrate: Add systemd service unit to trigger on boot

Note that the service is intentionally not enabled by default as it
should be up to the distribution to add a systemd preset if the
migration to a static GRUB config is needed.

This will be used on Atomic Desktops & IoT systems to migrate systems to
a static GRUB config before enabling composefs as GRUB curently does not
interact well with it:
https://bugzilla.redhat.com/show_bug.cgi?id=2308594

See: #789
See: https://fedoraproject.org/wiki/Changes/ComposefsAtomicDesktops

Makefile

@@ -43,6 +43,7 @@ install-grub-static:
 
 install-systemd-unit:
 	install -m 644 -D -t "${DESTDIR}$(PREFIX)/lib/systemd/system/" contrib/packaging/bootloader-update.service
+	install -m 644 -D -t "${DESTDIR}$(PREFIX)/lib/systemd/system/" contrib/packaging/bootupd-static-grub-migration.service
 
 bin-archive:
 	rm target/inst -rf

contrib/packaging/bootupd-static-grub-migration.service

@@ -0,0 +1,17 @@
+[Unit]
+Description=bootupd static GRUB config migration
+Documentation=https://github.com/coreos/bootupd
+ConditionPathExists=!/boot/.bootupd-static-migration-complete
+RequiresMountsFor=/sysroot /boot
+# Only run after a successful bootloader update
+After=bootloader-update.service
+Requires=bootloader-update.service
+
+[Service]
+Type=oneshot
+ExecStart=/usr/bin/bootupctl migrate
+RemainAfterExit=yes
+MountFlags=slave
+
+[Install]
+WantedBy=multi-user.target

contrib/packaging/bootupd.spec

@@ -36,6 +36,7 @@ BuildRequires: systemd-rpm-macros
 %{_libexecdir}/bootupd
 %{_prefix}/lib/bootupd/grub2-static/
 %{_unitdir}/bootloader-update.service
+%{_unitdir}/bootupd-static-grub-migration.service
 
 %prep
 %autosetup -n %{crate}-%{version} -p1 -Sgit