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>