rauc-conf.bbappend: WORKDIR -> UNPACKDIR transition

This adapts to the oe-core rework to enforce a separate directory
for unpacking local sources (UNPACKDIR) instead of directly using
WORKDIR.

Follows the preliminary guideline from:
https://lists.openembedded.org/g/openembedded-architecture/message/2007

Signed-off-by: Leon Anavi <[email protected]>

meta-rauc-raspberrypi/recipes-core/rauc/rauc-conf.bbappend

@@ -2,5 +2,5 @@ FILESEXTRAPATHS:prepend := "${THISDIR}/files:"
 SRC_URI:append := " file://ca.cert.pem "
 
 do_install:prepend() {
-	sed -i "s/@@MACHINE@@/${MACHINE}/g" ${WORKDIR}/system.conf
+	sed -i "s/@@MACHINE@@/${MACHINE}/g" ${S}/system.conf
 }