package/rpi-firmware: fix missing files in overlays

When supporting multiple hardware targets, overlay_map.dtb might
be needed to map overlay names to one of several implementations [1].
If the correct overlay names are specified in config.txt, the map file
is not needed, but it also doesn't hurt.

[1] https://github.com/raspberrypi/documentation/blob/develop/documentation/asciidoc/computers/configuration/device-tree.adoc#the-overlay-map-file

Signed-off-by: Rutger Sassen <rsassen@comecer.com>
Reviewed-by: Peter Seiderer <ps.report@gmx.net>
[Arnout: always install overlay_map.dtb]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
(cherry picked from commit da38cdead909aa133b9c468ddebf3f67c8d198f2)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/rpi-firmware/rpi-firmware.mk

@@ -48,6 +48,7 @@ define RPI_FIRMWARE_INSTALL_DTB_OVERLAYS
 	$(foreach ovldtb,$(wildcard $(@D)/boot/overlays/*.dtbo), \
 		$(INSTALL) -D -m 0644 $(ovldtb) $(BINARIES_DIR)/rpi-firmware/overlays/$(notdir $(ovldtb))
 	)
+	$(INSTALL) -D -m 0644 $(@D)/boot/overlays/overlay_map.dtb $(BINARIES_DIR)/rpi-firmware/overlays/
 endef
 else
 # Still create the directory, so a genimage.cfg can include it independently of