Fix missing rpi-rf-mod overlay on Yellow (#4244)

Before update to Buildroot 2025.02, the overlays directory on Yellow was
created by rpi-firmware in a condition added confusingly in firmware bump [1].
However, this got lost during Buildroot update, and since Yellow doesn't copy
overlays from the rpi-firmware repo, the directory was never created and the
rpi-rf-mod.dtbo couldn't be copied there in pre-image build hook.

To make things more robust, create the overlays directory for rpi targets
conditionally in the hook instead of relying on rpi-firmware to create it.

[1] f1af1a0bf7

Fixes #4233

buildroot-external/board/raspberrypi/hassos-hook.sh

@@ -19,7 +19,10 @@ function hassos_pre_image() {
     else
         cp "${BOARD_DIR}/../cmdline.txt" "${BOOT_DATA}/cmdline.txt"
     fi
-    cp "${BINARIES_DIR}"/*.dtbo "${BOOT_DATA}/overlays/" 2>/dev/null || true
+    if ls "${BINARIES_DIR}"/*.dtbo >/dev/null 2>&1; then
+        mkdir -p "${BOOT_DATA}/overlays"
+        cp "${BINARIES_DIR}"/*.dtbo "${BOOT_DATA}/overlays/"
+    fi
 
     # Enable 64bit support
     if [[ "${BOARD_ID}" =~ "64" ]]; then