rpi-u-boot-scr: use UNPACKDIR

* see:
  https://lists.openembedded.org/g/openembedded-architecture/message/2007

* fixes:
  sed: can't read TOPDIR/BUILD/work/raspberrypi4_64-webos-linux/rpi-u-boot-scr/1.0/boot.cmd.in: No such file or directory

Signed-off-by: Martin Jansa <martin.jansa@gmail.com>

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

@@ -15,7 +15,7 @@ do_compile() {
     sed -e 's/@@KERNEL_IMAGETYPE@@/${KERNEL_IMAGETYPE}/' \
         -e 's/@@KERNEL_BOOTCMD@@/${KERNEL_BOOTCMD}/' \
         -e 's/@@BOOT_MEDIA@@/${BOOT_MEDIA}/' \
-        "${WORKDIR}/boot.cmd.in" > "${WORKDIR}/boot.cmd"
+        "${UNPACKDIR}/boot.cmd.in" > "${WORKDIR}/boot.cmd"
     mkimage -A ${UBOOT_ARCH} -T script -C none -n "Boot script" -d "${WORKDIR}/boot.cmd" boot.scr
 }