-systemd.bbclass incorrect for nativesdk packages

We should use the 'bare' package name BPN instead of PN for finding the
service file.  Otherwise, a -nativesdk package cannot be created because
we get a file-not-found error on the service file.
diff --git a/meta-phosphor/classes/obmc-phosphor-systemd.bbclass b/meta-phosphor/classes/obmc-phosphor-systemd.bbclass
index 7bd2d9d..12e0bc1 100644
--- a/meta-phosphor/classes/obmc-phosphor-systemd.bbclass
+++ b/meta-phosphor/classes/obmc-phosphor-systemd.bbclass
@@ -2,13 +2,13 @@
 
 inherit systemd
 
-SYSTEMD_SERVICE_${PN} = "${PN}.service"
+SYSTEMD_SERVICE_${PN} = "${BPN}.service"
 SRC_URI += " \
-        file://${PN}.service \
+        file://${BPN}.service \
         "
 
 do_install_append() {
         # install systemd unit files
         install -d ${D}${systemd_unitdir}/system
-        install -m 0644 ${WORKDIR}/${PN}.service ${D}${systemd_unitdir}/system
+        install -m 0644 ${WORKDIR}/${BPN}.service ${D}${systemd_unitdir}/system
 }