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-phosphor/recipes-phosphor/video/fbterm_git.bb b/meta-phosphor/recipes-phosphor/video/fbterm_git.bb
index 57e2174..8480bc6 100644
--- a/meta-phosphor/recipes-phosphor/video/fbterm_git.bb
+++ b/meta-phosphor/recipes-phosphor/video/fbterm_git.bb
@@ -26,4 +26,4 @@
 }
 
 SYSTEMD_SERVICE:${PN} += "fbterm.service"
-SYSTEMD_ENVIRONMENT_FILE_${PN} += "fbterm"
+SYSTEMD_ENVIRONMENT_FILE:${PN} += "fbterm"