ostree-prepare-root.service: Use RemainAfterExit=yes

For the same reasons as #1697. This is especially important in services
that are likely to be used as an `After/Before=` target in other units.
`ostree-prepare-root.service` is one such service.

Closes: #1759
Approved by: cgwalters

src/boot/ostree-prepare-root.service

@@ -32,3 +32,4 @@ ExecStart=/usr/lib/ostree/ostree-prepare-root /sysroot
 StandardInput=null
 StandardOutput=syslog
 StandardError=syslog+console
+RemainAfterExit=yes