boot: Use emergency.target, not emergency.service

Follow systemd units in using emergency.target, not emergency.service
(which is the sole unit, by default, in emergency.target) so we can
easily reconfigure the units which are actived when entering
emergency mode.

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>

Closes: #1665
Approved by: cgwalters

src/boot/ostree-prepare-root.service

@@ -20,7 +20,7 @@ Description=OSTree Prepare OS/
 DefaultDependencies=no
 ConditionKernelCommandLine=ostree
 ConditionPathExists=/etc/initrd-release
-OnFailure=emergency.service
+OnFailure=emergency.target
 After=initrd-switch-root.target
 Before=initrd-switch-root.service
 Before=plymouth-switch-root.service

src/boot/ostree-remount.service

@@ -19,7 +19,7 @@
 Description=OSTree Remount OS/ bind mounts
 DefaultDependencies=no
 ConditionKernelCommandLine=ostree
-OnFailure=emergency.service
+OnFailure=emergency.target
 Conflicts=umount.target
 After=-.mount
 After=systemd-remount-fs.service