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/dbus/phosphor-dbus-monitor_%.bbappend b/meta-ibm/recipes-phosphor/dbus/phosphor-dbus-monitor_%.bbappend
index 3071cb4..6731a7a 100644
--- a/meta-ibm/recipes-phosphor/dbus/phosphor-dbus-monitor_%.bbappend
+++ b/meta-ibm/recipes-phosphor/dbus/phosphor-dbus-monitor_%.bbappend
@@ -1,2 +1,2 @@
-SYSTEMD_LINK_phosphor-msl-verify:append:ibm-ac-server = " ../phosphor-msl-verify.service:obmc-chassis-poweron@0.target.requires/phosphor-msl-verify.service"
-SYSTEMD_LINK_phosphor-msl-verify:append:mihawk = " ../phosphor-msl-verify.service:obmc-chassis-poweron@0.target.requires/phosphor-msl-verify.service"
+SYSTEMD_LINK:phosphor-msl-verify:append:ibm-ac-server = " ../phosphor-msl-verify.service:obmc-chassis-poweron@0.target.requires/phosphor-msl-verify.service"
+SYSTEMD_LINK:phosphor-msl-verify:append:mihawk = " ../phosphor-msl-verify.service:obmc-chassis-poweron@0.target.requires/phosphor-msl-verify.service"