In version 1.1.1, the new install target in the Makefile was used to
install rmtfs. This installs rmtfs to /usr/bin instead of the expected
/usr/sbin. Use the new /usr/bin directory for the OpenRC service file so
rmtfs can keep working under OpenRC.
Fixes: f7052461da ("modem/rmtfs: subpackage -systemd")
Part-of: <https://gitlab.postmarketos.org/postmarketOS/pmaports/-/merge_requests/7517>