Yocto 5.1 (styhead): S = ${WORKDIR} no longer supported

Fix errors during parse due to S = ${WORKDIR}
https://docs.yoctoproject.org/next/migration-guides/migration-5.1.html#workdir-changes :

    S = ${WORKDIR} no longer supported

    If a recipe has S set to be WORKDIR, this is no longer supported,
    and an error will be issued. The recipe should be changed to:

    S = "${WORKDIR}/sources"
    UNPACKDIR = "${S}"

    Any WORKDIR references where files from SRC_URI are referenced
    should be changed to S. These are commonly in do_compile,
    do_compile, do_install and LIC_FILES_CHKSUM.

Signed-off-by: Geoff Parker <geoffrey.parker@arthrex.com>

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

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