Adding device tree overlay in yocto for raspberry pi

I am trying to add a device tree overlay file in yocto for the raspberry pi. I have read this answer however there are still some things I am not understanding.

1. I have created a workspace using dev-tool workspace/sources/linux-raspberrypi/
2. The answer above says place the dts file in ${TOPDIR}/workspace/sources/linux-raspberrypi/, however on my system this folder just contains sub folders containing dst and dsti files. It also contains a sub folder called overlays/ this seems a better place to me?
3. The answer above also says you should add you dts file to the Makefile however I am unsure what to add the file. In dts/overlays/Makefile there is just a list of dtbo files, no dts files e.g.

# Overlays for the Raspberry Pi platform

dtb-$(CONFIG_ARCH_BCM2835) += overlay_map.dtb hat_map.dtb

dtbo-$(CONFIG_ARCH_BCM2835) += \
        act-led.dtbo \
        adafruit-st7735r.dtbo \
...
        wm8960-soundcard.dtbo

targets += dtbs dtbs_install
targets += $(dtbo-y)

always-y        := $(dtbo-y)
clean-files     := *.dtbo

so how do I add my dts file? I have been using dtc -@ -Hepapr -I dts -O dtb -o myOverlayFile.dtbo myOverlayFile.dts when manually compiling my dts files so was expecting something similar.

4. The answer above talks about adding to KERNEL_DEVICETREE in conf/machine/custom-machine.conf (is it literally "custom-machine" or doe it need to match MACHINE variable) however when I look at bitbake -e linux-raspberrypi | grep "KERNEL_DEVICETREE" It also shows what looks like what is an interesting variable RPI_KERNEL_DEVICETREE_OVERLAY, should the dts file be added to this variable in conf/machine/custom-machine.conf as well or instead of KERNEL_DEVICETREE?

Version info

$ bitbake --version
BitBake Build Tool Core version 2.9.1
$ bitbake -e | grep '^DISTRO_VERSION\|^DISTRO_CODENAME='
DISTRO_CODENAME="styhead"
DISTRO_VERSION="5.1.2"