Without seeing the code, it’s hard to say for sure, but my bet is that there’s a typo in the extra-leds-dts.mk file since I do that frequently. Maybe run make extra-leds-dts-rebuild to see or look for the build directory.

Having said that, I have a couple things:

1. If you want the dtbo to be in the images directory, check out the Buildroot docs for $(BINARIES_DIR), but I’d copy to a different directory from the rpi-firmware package’s output directory. I’d probably just put it in the root images directory if it’s only one file.
2. If you want the Raspberry Pi bootloader to load the dtbo, you’ll need to update the config.txt as well. That seems to be the path you’re going down.
3. If your extra LEDs can wait to be initialized at runtime, I’d load the overlay at runtime using the dtoverlay program. I’d also compile your overlay as part of your Elixir project and put the .dtbo in a priv directory so there’s nothing you need to do with Buildroot or Nerves Systems at all.

Hope that helps.

Update:

I am able to compile now but still can’t get the dtbo file to land somewhere I can use it.

To enable compiling I had to add external.mk to the nerves system like the bbb system has.

So the outstanding issue is getting

define MM_FSERIES_DTS_INSTALL_TARGET_CMDS
cp $(@D)/*.dtbo $(TARGET_DIR)/images/rpi-firmware/overlays/
endef

The TARGET_DIR is not appropriate but cannot figure out what variable to use

Finally got this working. Thanks @fhunleth for the assistance on finding the correct variable.

To make this work you have to:

1. add external.mk file to the root of your pi system

# Include system-specific packages
include $(sort $(wildcard $(NERVES_DEFCONFIG_DIR)/package/*/*.mk))

2. Create package dir at the root of your pi system
3. Create your Buildroot Package (See nerves_system_bbb for an example)
4. Add package to package_files/0 in the mix.exs file. When changes in the version occur it will be rebuilt.
5. Add your custom package to nerves_defconfig
6. Update your fwup.conf to include the new dtbo (see below)
7. Update your config.txt to load the new overlay

Here is a complete Makefile for a custom device tree overlay

#############################################################
#
# custom-dts
#
#############################################################

# Remember to bump the version when anything changes in this
# directory.
CUSTOM_DTS_SOURCE =
CUSTOM_DTS_VERSION = 0.1.0
CUSTOM_DTS_DEPENDENCIES = host-dtc

define CUSTOM_DTS_BUILD_CMDS
	cp $(NERVES_DEFCONFIG_DIR)/package/custom-dts/*.dts* $(@D)
        for filename in $(@D)/*.dts; do \
            $(CPP) -I$(@D) -I $(LINUX_SRCDIR)include -I $(LINUX_SRCDIR)arch -nostdinc -undef -D__DTS__ -x assembler-with-cpp $$filename | \
              $(HOST_DIR)/usr/bin/dtc -Wno-unit_address_vs_reg -@ -I dts -O dtb -b 0 -o $${filename%.dts}.dtbo || exit 1; \
        done
endef

define CUSTOM_DTS_INSTALL_TARGET_CMDS
        cp $(@D)/*.dtbo $(BINARIES_DIR)/rpi-firmware/overlays/
endef


$(eval $(generic-package))

Then in your fwup.conf file you can access this file with

file-resource custom.dtbo {
    host-path = "${NERVES_SYSTEM}/images/rpi-firmware/overlays/custom.dtbo"
}

Then in your config.txt add: dtoverlay=custom