feat: allow empty Memfault project key

[gminn]

Summary

In cases where an upstream device uploads data to Memfault on behalf
of a downstream device, the project key is not required. This change
allows the project key to be optional in those cases. For the
NCS Memfault sources, this is when we are not using MDS or the
Memfault Zephyr HTTP client.

Test Plan

Confirmed that before this change, the build fails if the project key
is not set for a generic cortex-m application where Memfault is enabled:

west build -b qemu_cortex_m3
zephyr/samples/hello_world -- \
-DCONFIG_MEMFAULT=y \
-DCONFIG_MEMFAULT_NCS_FW_TYPE=\"test-app\" \
-DCONFIG_MEMFAULT_NCS_DEVICE_ID=\"my-test-device\"

Confirmed after this change, the build succeeds with this same build command.

Also confirmed that after this change, the build still fails if the project key
is not set for projects where MDS or the Memfault Zephyr HTTP client
are used:

west build -b nrf52840dk/nrf52840 \
nrf/samples/bluetooth/peripheral_mds
...
/Users/gminnehan/projects/memfault-sdk-nrf-workspace/zephyr/include/zephyr/toolchain/gcc.h:87:36: error: static assertion failed: "Memfault Project Key not configured. Please visit https://goto.memfault.com/create-key/nrf "
 87 | #define BUILD_ASSERT(EXPR, MSG...) _Static_assert((EXPR), "" MSG)
    |                                    ^~~~~~~~~~~~~~
/Users/gminnehan/projects/memfault-sdk-nrf-workspace/nrf/modules/memfault-firmware-sdk/memfault_integration.c:40:1: note: in expansion of macro 'BUILD_ASSERT'
 40 | BUILD_ASSERT(sizeof(CONFIG_MEMFAULT_NCS_PROJECT_KEY) > 1,
    | ^~~~~~~~~~~~

west build -b thingy91/nrf9160/ns -p always \
nrf/applications/asset_tracker_v2 -- \
-DOVERLAY_CONFIG=overlay-memfault.conf
...
/Users/gminnehan/projects/memfault-sdk-nrf-workspace/zephyr/include/zephyr/toolchain/gcc.h:87:36: error: static assertion failed: "Memfault Project Key not configured. Please visit https://goto.memfault.com/create-key/nrf91 "
 87 | #define BUILD_ASSERT(EXPR, MSG...) _Static_assert((EXPR), "" MSG)
    |                                    ^~~~~~~~~~~~~~
/Users/gminnehan/projects/memfault-sdk-nrf-workspace/nrf/modules/memfault-firmware-sdk/memfault_integration.c:40:1: note: in expansion of macro 'BUILD_ASSERT'
 40 | BUILD_ASSERT(sizeof(CONFIG_MEMFAULT_NCS_PROJECT_KEY) > 1,
    | ^~~~~~~~~~~~

and still succeeds when the project key is passed:

west build -b nrf52840dk/nrf52840 \
nrf/samples/bluetooth/peripheral_mds -- \
-DCONFIG_MEMFAULT_NCS_PROJECT_KEY=\"$(<~/.memfault-gilly-playground-proj-key)\"

west build -b thingy91/nrf9160/ns \
nrf/applications/asset_tracker_v2 -- \
-DCONFIG_MEMFAULT_NCS_PROJECT_KEY=\"$(<~/.memfault-gilly-playground-proj-key)\" \
-DOVERLAY_CONFIG=overlay-memfault.conf

---

Resolve: MCU-982

[gminn]

• feat: allow empty Memfault project key #31 👈 (View in Graphite)
• main

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[noahp]

thank you! i think this is the simplest approach- we do have a built-in structure now, but to avoid messing up existing configurations, i think it's easiest to make the change you made here 👍

https://github.com/memfault/memfault-firmware-sdk/blob/f1a88a848b4f0a634375ab488d2d1f2a52085bfa/ports/zephyr/common/memfault_platform_http.c#L80-L90

[gminn]

Merged upstream at nrfconnect#21559