core: build_id: support alternate GNU build ID sources

[JordanYates]

External build systems may already be enabling GNU build ID, in which case the Memfault provided implementation either breaks the upstream version or doesn't work.

CONFIG_MEMFAULT_PLATFORM_CONFIG enables users to override the configuration file if they desire.

By setting CONFIG_MEMFAULT_GNU_BUILD_ID_EXTERNAL=y and appropriately configuring MEMFAULT_GNU_BUILD_ID_SYMBOL, memfault_build_id.c will pick up whatever symbol is being created by the build system.

This enables integration with solutions such as zephyrproject-rtos/zephyr#51532, and presumably the final result of zephyrproject-rtos/zephyr#54464

[JordanYates]

A .elf file created with this PR linking to an alternate symbol (__g_note_build_id) and manually uploaded to Memfault cloud works as expected:

[ejohnso49]

@JordanYates I'm working on these changes right now and I have a couple of questions:

• For your example of linking to an alternate symbol, did you use your own linker script fragment similar to ports/zephyr/common/memfault-build-id.ld?
• Could you explain the need to override memfault_zephyr_platform_config.h? Why not use the user-defined memfault_platform_config.h to override any values?

[JordanYates]

My implementation is the same as what I proposed upstreaming here: zephyrproject-rtos/zephyr#51532.

For the config's I possibly am confused about which configs are applying where, there are many versions in the tree to keep track of :) I thought memfault_zephyr_platform_config.h was the version of memfault_platform_config.h used when compiling with Zephyr?

[ejohnso49]

For the config's I possibly am confused about which configs are applying where, there are many versions in the tree to keep track of :) I thought memfault_zephyr_platform_config.h was the version of memfault_platform_config.h used when compiling with Zephyr?

Probably means we could add some documentation around how these layer together. memfault_platform_config.h is intended for your application's config and overrides default SDK values. We could use some improvement in where this is placed and the file name but in general that's where you should override values like MEMFAULT_GNU_BUILD_ID_SYMBOL.

My implementation is the same as what I proposed upstreaming here: zephyrproject-rtos/zephyr#51532.

Great, I'm working on getting a PR for this review soon. Hope to have this included with our next release. I opted to build a menu around the build ID options

[JordanYates]

Integrated with v1.1.3