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-ingrasys/meta-zaius/recipes-phosphor/chassis/avsbus-control.bb b/meta-ingrasys/meta-zaius/recipes-phosphor/chassis/avsbus-control.bb
index 4aebf6f..b272dfe 100644
--- a/meta-ingrasys/meta-zaius/recipes-phosphor/chassis/avsbus-control.bb
+++ b/meta-ingrasys/meta-zaius/recipes-phosphor/chassis/avsbus-control.bb
@@ -16,9 +16,9 @@
 FMT_ON = "../${TMPL_ON}:${TGTFMT_ON}.requires/${INSTFMT_ON}"
 
 SYSTEMD_SERVICE:${PN} += "${TMPL_OFF}"
-SYSTEMD_LINK_${PN} += "${@compose_list(d, 'FMT_OFF', 'OBMC_CHASSIS_INSTANCES')}"
+SYSTEMD_LINK:${PN} += "${@compose_list(d, 'FMT_OFF', 'OBMC_CHASSIS_INSTANCES')}"
 SYSTEMD_SERVICE:${PN} += "${TMPL_ON}"
-SYSTEMD_LINK_${PN} += "${@compose_list(d, 'FMT_ON', 'OBMC_CHASSIS_INSTANCES')}"
+SYSTEMD_LINK:${PN} += "${@compose_list(d, 'FMT_ON', 'OBMC_CHASSIS_INSTANCES')}"
 
 SRC_URI += "file://zaius_avsbus.sh"
 RDEPENDS:${PN} += "i2c-tools"