[FEATURE] drop git submodules and switch to Zephyr's "west" multi-repo tool

[marc-hb]

Rationale, pros and cons: https://docs.google.com/document/d/1TuJ8Z4W9rr1vUuKQ0O3Vh5dPPnGw5dKizylaEPtiPcg

I suggest having questions and discussions about implementation details in this Google doc and higher level discussions here. Whatever works.

Related:

• Continuous Integration with multiple git repos? (using west or similar) zephyrproject-rtos/zephyr#34713
• Transfer rimage git history to subdirectory tools/rimage/ in the sof git repo #8297

[lgirdwood]

@marc-hb lets make sure this can work with Zephyr west and without. i.e. when we are using Zephyr or using xtos. Lets also mark this for v1.7

[marc-hb]

This should solve the problem of synchronizing multi-repo submissions like this kconfig one: thesofproject/sof-test#458

[marc-hb]

Let's make sure this can work with Zephyr west and without. i.e. when we are using Zephyr or using xtos.

This shouldn't be a problem.

I think the next step is to propose a hierarchy of (optional) manifests. In addition to the above requirement, we also don't want to force every single firmware developer to download a Linux kernel repo that is 50 times bigger than the firmware repo they need and that most people will never use. This would also slow down many CI checks; CI code frequently git clones from scratch because for small repos it doesn't matter.

Note shallow --depth support in west is... complicated: zephyrproject-rtos/west#319

[lgirdwood]

@andyross @nashif fyi

[lgirdwood]

Moving t v1.9 as this will occur over time as Zephyr is integrated.