Add simple service to comment / in fstab

ostree modes have conflictings / mount needs:
- "hardlinks" mode need / to be rw
- composefs mode need / to be ro

Some installation methods (at least Anaconda) add / to fstab,
so when systemd-remount-fs.service tries to remount
the composefs / rw, it fails because it can only be ro.

To be able to edit /etc this early during the boot it rely on
having the 'rw' kargs.

bootc has a systemd generator to edit fstab but not everyone uses bootc (yet).

Makefile-boot.am

@@ -42,6 +42,7 @@ systemdsystemunit_DATA = src/boot/ostree-prepare-root.service \
 	src/boot/ostree-finalize-staged.service \
 	src/boot/ostree-finalize-staged.path \
 	src/boot/ostree-finalize-staged-hold.service \
+	src/boot/ostree-edit-fstab.service \
 	src/boot/[email protected] \
 	$(NULL)
 systemdtmpfilesdir = $(prefix)/lib/tmpfiles.d

src/boot/ostree-edit-fstab.service

@@ -0,0 +1,30 @@
+# This library is free software; you can redistribute it and/or
+# modify it under the terms of the GNU Lesser General Public
+# License as published by the Free Software Foundation; either
+# version 2 of the License, or (at your option) any later version.
+#
+# This library is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+# Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public
+# License along with this library. If not, see <https://www.gnu.org/licenses/>.
+
+[Unit]
+Description=OSTree comment / in fstab
+DefaultDependencies=no
+ConditionKernelCommandLine=ostree
+Conflicts=shutdown.target
+After=systemd-fsck-root.service
+Before=bootc-fstab-edit.service systemd-remount-fs.service local-fs-pre.target local-fs.target shutdown.target
+Wants=local-fs-pre.target
+ConditionPathIsReadWrite=/etc
+
+[Service]
+Type=oneshot
+RemainAfterExit=yes
+ExecStart=/usr/bin/sed -i 's/^\\([^#[:space:]]\\+[[:space:]]\\+\\/[[:space:]]\\+\\)/# Avoid \\/ being remounted by systemd-remount-fs.service (ostree-edit-fstab.service)\\n#\\1/' /etc/fstab
+
+[Install]
+WantedBy=local-fs.target