depends: Add 'make clean' and 'make clean-all' rules
depends: Add 'make clean' and 'make clean-all' rules

It's useful to have a standard way to clean up the work done by the
depends system when testing changes to it.

The `make clean-all` rule removes build artifacts for all
supported architectures (in addition to sources/), while `make clean`
only removes artifacts for current architecture (`BUILD`).

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