Fix rasberry Pi 64bit firmware overlay inclusion

Fix rasberry Pi 64bit firmware overlay inclusion

This patch enables the inclusion of the Pi's overlays. Previously
the overlays were not included in the genimage configuration.
This patch ensures overlays are included in the sdcard (when
enabled) by defaulting to the inclusion of an empty
output/images/rpi-firmware/overlays directory in genimage cfg.

The Pi's overlays are built with the following config
variables:
BR2_PACKAGE_RPI_FIRMWARE=y
BR2_PACKAGE_RPI_FIRMWARE_INSTALL_DTBS=y
BR2_PACKAGE_RPI_FIRMWARE_INSTALL_DTB_OVERLAYS=y
BR2_LINUX_KERNEL_IMAGE_TARGET_CUSTOM=y
BR2_LINUX_KERNEL_IMAGE_NAME="Image"
BR2_LINUX_KERNEL_IMAGE_TARGET_NAME="Image modules dtbs"

After building, the dtbo files are present in the
output/images/rpi-firmware/overlays directory but not added
to the sdcard because they are missing from the genimage cfg
file.

Signed-off-by: Matt Flax <flatmax@flatmax.org>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[Arnout: fix indentation, and add comment explaining why an empty
         directory is created.]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>