This simplifies the dependencies:
1. Objs depend on headers, not other objs.
2. Programs depend on objs.
3. A .o file will generally implicitly depend on the .c file it's built from.
4. If a file has a build line, it's often better to list all deps there.
5. I spotted some missing 'make clean' files.
The particular problem in this case seems to be that make would use
tools/test/gen_test.c before it was ready. It's probably confused by
the use of recursive make via update-mocks, so explicitly split that
into two stages.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
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