cpuflpr target broken on custom nRF54L15 boards using sysbuild

[gmarull]

Describe the bug

When creating a custom board based on nRF54L15, the build is broken for cpuflpr target if using sysbuild. This is likely because the flpr snippets are not populated for such board, but can hardly be achieved downstream (where custom boards usually reside).

To Reproduce

• Create a custom board for nRF54L15, e.g. with: west ncs-genboard -o boards -e "myvnd" -b myboard -d "My Board" -s nrf54l15 -v qfaa -n "2.7.0" from https://github.com/nrfconnect/sdk-nrf/pull/16861
• Run west build -b myboard/nrf54l15/cpuflpr samples/hello_world -p --sysbuild
• Observe the failures

   **********************************
   * Running CMake for vpr_launcher *
   **********************************

Loading Zephyr default modules (Zephyr base).
-- Application: /home/gmarull/ws/nordic/zephyrproject/zephyr/samples/basic/minimal
-- CMake version: 3.28.2
-- Found Python3: /home/gmarull/ws/nordic/zephyrproject/.venv/bin/python (found suitable version "3.12.4", minimum required is "3.8") found components: Interpreter 
-- Cache files will be written to: /home/gmarull/.cache/zephyr
-- Zephyr version: 3.7.99 (/home/gmarull/ws/nordic/zephyrproject/zephyr)
-- Found west (found suitable version "1.2.0", minimum required is "0.14.0")
-- Board: myboard, qualifiers: nrf54l15/cpuapp
-- Snippet(s): nordic-flpr
-- ZEPHYR_TOOLCHAIN_VARIANT not set, trying to locate Zephyr SDK
-- Found host-tools: zephyr 0.16.8 (/home/gmarull/zephyr-sdk-0.16.8)
-- Found toolchain: zephyr 0.16.8 (/home/gmarull/zephyr-sdk-0.16.8)
-- Found Dtc: /home/gmarull/zephyr-sdk-0.16.8/sysroots/x86_64-pokysdk-linux/usr/bin/dtc (found suitable version "1.6.0", minimum required is "1.4.6") 
-- Found BOARD.dts: /home/gmarull/ws/nordic/zephyrproject/zephyr/boards/myvnd/myboard/myboard_nrf54l15_cpuapp.dts
-- Found devicetree overlay: /home/gmarull/ws/nordic/zephyrproject/zephyr/snippets/nordic-flpr/nordic-flpr.overlay
-- In: /home/gmarull/ws/nordic/zephyrproject/zephyr/build/vpr_launcher/zephyr, command: /home/gmarull/ws/nordic/zephyrproject/.venv/bin/python;/home/gmarull/ws/nordic/zephyrproject/zephyr/scripts/dts/gen_defines.py;--dts;/home/gmarull/ws/nordic/zephyrproject/zephyr/build/vpr_launcher/zephyr/zephyr.dts.pre;--dtc-flags;'-Wno-unique_unit_address_if_enabled';--bindings-dirs;/home/gmarull/ws/nordic/zephyrproject/zephyr/dts/bindings;--header-out;/home/gmarull/ws/nordic/zephyrproject/zephyr/build/vpr_launcher/zephyr/include/generated/zephyr/devicetree_generated.h.new;--dts-out;/home/gmarull/ws/nordic/zephyrproject/zephyr/build/vpr_launcher/zephyr/zephyr.dts.new;--edt-pickle-out;/home/gmarull/ws/nordic/zephyrproject/zephyr/build/vpr_launcher/zephyr/edt.pickle;--vendor-prefixes;/home/gmarull/ws/nordic/zephyrproject/zephyr/dts/bindings/vendor-prefixes.txt
CMake Error at /home/gmarull/ws/nordic/zephyrproject/zephyr/cmake/modules/dts.cmake:311 (message):
  gen_defines.py failed with result code: 1 - stderr contents:

  devicetree error: 'execution-memory' is marked as required in 'properties:'
  in
  /home/gmarull/ws/nordic/zephyrproject/zephyr/dts/bindings/riscv/nordic,nrf-vpr-coprocessor.yaml,
  but does not appear in <Node /soc/peripheral@50000000/vpr@4c000 in
  '/home/gmarull/ws/nordic/zephyrproject/zephyr/misc/empty_file.c'>

Call Stack (most recent call first):
  /home/gmarull/ws/nordic/zephyrproject/zephyr/cmake/modules/zephyr_default.cmake:133 (include)
  /home/gmarull/ws/nordic/zephyrproject/zephyr/share/zephyr-package/cmake/ZephyrConfig.cmake:66 (include)
  /home/gmarull/ws/nordic/zephyrproject/zephyr/share/zephyr-package/cmake/ZephyrConfig.cmake:92 (include_boilerplate)
  CMakeLists.txt:5 (find_package)

-- Configuring incomplete, errors occurred!
CMake Error at cmake/modules/sysbuild_extensions.cmake:486 (message):
  CMake configure failed for Zephyr project: vpr_launcher

  Location:
  /home/gmarull/ws/nordic/zephyrproject/zephyr/samples/basic/minimal
Call Stack (most recent call first):
  cmake/modules/sysbuild_images.cmake:20 (ExternalZephyrProject_Cmake)
  cmake/modules/sysbuild_default.cmake:20 (include)
  /home/gmarull/ws/nordic/zephyrproject/zephyr/share/zephyr-package/cmake/ZephyrConfig.cmake:75 (include)
  /home/gmarull/ws/nordic/zephyrproject/zephyr/share/zephyr-package/cmake/ZephyrConfig.cmake:92 (include_boilerplate)
  /home/gmarull/ws/nordic/zephyrproject/zephyr/share/sysbuild-package/cmake/SysbuildConfig.cmake:8 (include)
  template/CMakeLists.txt:10 (find_package)

Expected behavior

Out of tree boards should work out of the box, or have a mechanism to make them work.

Impact

Blocker for out of tree nRF54L15 based boards.

Logs and console output

See above.

Environment (please complete the following information):

• OS: Linux
• Toolchain: Zephyr SDK 0.16.8
• Commit SHA or Version used 7b4e7ff9eda0227762666a39636b5f7729fdfd51

Additional context

N/A

[gmarull]

Note that wildcards in snippet may be a solution if the overlay can be applied universally to all nRF54L15 boards.

[nordic-piks]

@barsok Please have a look, this is probably a new feature request.

[nordic-piks]

Triaged internally