Fix regression in nondeterministic failure of overlay services (#620)

This PR attempts to fix the problem described in
#618 (comment)
(and analyzed in
#618 (comment)),
which may be a regression caused by #613.

Author: ethanjli

software/distro/setup/base-os/forklift/usr/lib/systemd/system/overlay-etc.service

@@ -1,6 +1,10 @@
 [Unit]
 Description=Remount /etc as a writable overlay with a Forklift-managed intermediate layer, during early boot
 DefaultDependencies=no
+Requires=-.mount
+After=-.mount
+Requires=systemd-remount-fs.service
+After=systemd-remount-fs.service
 Wants=systemd-machine-id-commit.service
 After=systemd-machine-id-commit.service
 # If forklift (or the upperdir) provides kernel parameters via `/etc/sysctl.d`, we must make those

software/distro/setup/base-os/forklift/usr/lib/systemd/system/overlay-usr.service

@@ -1,6 +1,10 @@
 [Unit]
 Description=Remount /usr as a writable overlay with a Forklift-managed intermediate layer, during early boot
 DefaultDependencies=no
+Requires=-.mount
+After=-.mount
+Requires=systemd-remount-fs.service
+After=systemd-remount-fs.service
 Wants=overlay-fs.target
 Before=overlay-fs.target
 Conflicts=umount.target