meta-phosphor: change systemd.bbclass to match upstream override syntax

Upstream bbclasses changed to typically use the `:${PN}` override
syntax, including the SYSTEMD_ variables.  Change our systemd.bbclass
to do the same for consistency and perform a tree-wide variable
replacement.

Spot checked by building bletchley and witherspoon and checking
some of the SYSTEMD_LINK directives on installed packages under qemu.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I20a9dd809bff8af8759488734f80486c7228c6eb
diff --git a/meta-ibm/recipes-phosphor/fans/fan-watchdog.bb b/meta-ibm/recipes-phosphor/fans/fan-watchdog.bb
index ad44ba3..4673c44 100644
--- a/meta-ibm/recipes-phosphor/fans/fan-watchdog.bb
+++ b/meta-ibm/recipes-phosphor/fans/fan-watchdog.bb
@@ -18,6 +18,6 @@
 WATCHDOG_FMT = "../${WATCHDOG_SERVICE}:${TGTFMT}.requires/${WATCHDOG_SERVICE}"
 
 SYSTEMD_SERVICE:${PN} += "${RESET_SERVICE} ${MONITOR_SERVICE} ${WATCHDOG_SERVICE}"
-SYSTEMD_LINK_${PN} += "${RESET_FMT} ${WATCHDOG_FMT}"
-SYSTEMD_ENVIRONMENT_FILE_${PN} += "obmc/fan-watchdog/reset-fan-watchdog.conf"
-SYSTEMD_ENVIRONMENT_FILE_${PN} += "obmc/fan-watchdog/fan-watchdog.conf"
+SYSTEMD_LINK:${PN} += "${RESET_FMT} ${WATCHDOG_FMT}"
+SYSTEMD_ENVIRONMENT_FILE:${PN} += "obmc/fan-watchdog/reset-fan-watchdog.conf"
+SYSTEMD_ENVIRONMENT_FILE:${PN} += "obmc/fan-watchdog/fan-watchdog.conf"