initscripts-swupdate-usb: fix S on scarthgap

Commit f1266f6 ported the recipe to styhead, where files are unpacked
into UNPACKDIR. Scarthgap does not know UNPACKDIR and always unpacks
into WORKDIR, so rcS.swupdate is not found in ${WORKDIR}/sources and
do_install fails.

Restore S = "${WORKDIR}", which is valid in scarthgap.

Signed-off-by: Stefano Babic <stefano.babic@swupdate.org>

recipes-core/initscripts-swupdate/initscripts-swupdate-usb.bb

@@ -7,8 +7,7 @@ LIC_FILES_CHKSUM = "file://${COMMON_LICENSE_DIR}/MIT;md5=0835ade698e0bcf8506ecda
 SRC_URI = "file://rcS.swupdate \
 	"
 
-S = "${WORKDIR}/sources"
-UNPACKDIR = "${S}"
+S = "${WORKDIR}"
 
 inherit allarch update-alternatives