rpi-firmware: change overlay file suffix from dtb to dtbo

Commit dc7b1c0 (rpi-firmware: bump version) bumped rpi-firmware to a version
based on kernel 4.4.x, but didn't update the overlay logic to handle the
.dtbo files from dynamic device tree support.

For details, see:
https://www.raspberrypi.org/forums/viewtopic.php?f=107&t=139732

[Peter: extend description]
Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/rpi-firmware/rpi-firmware.mk

@@ -23,7 +23,7 @@ endif
 
 ifeq ($(BR2_PACKAGE_RPI_FIRMWARE_INSTALL_DTB_OVERLAYS),y)
 define RPI_FIRMWARE_INSTALL_DTB_OVERLAYS
-	for ovldtb in  $(@D)/boot/overlays/*.dtb; do \
+	for ovldtb in  $(@D)/boot/overlays/*.dtbo; do \
 		$(INSTALL) -D -m 0644 $${ovldtb} $(BINARIES_DIR)/rpi-firmware/overlays/$${ovldtb##*/} || exit 1; \
 	done
 endef