rpi-u-boot-scr: 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 <leon.anavi@konsulko.com>

recipes-bsp/rpi-u-boot-scr/rpi-u-boot-scr.bb

@@ -21,6 +21,9 @@ do_compile() {
 
 inherit kernel-arch deploy nopackages
 
+S = "${WORKDIR}/sources"
+UNPACKDIR = "${S}"
+
 do_deploy() {
     install -d ${DEPLOYDIR}
     install -m 0644 boot.scr ${DEPLOYDIR}