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-openpower/recipes-phosphor/flash/phosphor-software-manager_%.bbappend b/meta-openpower/recipes-phosphor/flash/phosphor-software-manager_%.bbappend
index 5255be4..9df8193 100644
--- a/meta-openpower/recipes-phosphor/flash/phosphor-software-manager_%.bbappend
+++ b/meta-openpower/recipes-phosphor/flash/phosphor-software-manager_%.bbappend
@@ -1,4 +1,4 @@
 FILESEXTRAPATHS:prepend:df-openpower := "${THISDIR}/${PN}:"
 
-SYSTEMD_OVERRIDE_${PN}-updater:append:df-openpower = \
+SYSTEMD_OVERRIDE:${PN}-updater:append:df-openpower = \
     " software-bmc-updater.conf:xyz.openbmc_project.Software.BMC.Updater.service.d/software-bmc-updater.conf"