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-inspur/meta-on5263m5/recipes-phosphor/sensors/phosphor-hwmon_%.bbappend b/meta-inspur/meta-on5263m5/recipes-phosphor/sensors/phosphor-hwmon_%.bbappend
index 867c388..863f59c 100644
--- a/meta-inspur/meta-on5263m5/recipes-phosphor/sensors/phosphor-hwmon_%.bbappend
+++ b/meta-inspur/meta-on5263m5/recipes-phosphor/sensors/phosphor-hwmon_%.bbappend
@@ -16,4 +16,4 @@
 
 
 ENVS = "obmc/hwmon/{0}"
-SYSTEMD_ENVIRONMENT_FILE_${PN} += "${@compose_list(d, 'ENVS', 'ITEMS')}"
+SYSTEMD_ENVIRONMENT_FILE:${PN} += "${@compose_list(d, 'ENVS', 'ITEMS')}"