[PATCH] build: Fix dependencies for start and asm files

[Sebastian Huber]

Hello,

On 17/09/2020 07:57, Sebastian Huber wrote:
> Add explicit asm target feature.  Add a build start file node list and
> use it as a test program dependency.
>
> Close #3846.
> Close #4080.

I only have a vague feeling what these changes do. Thomas Nagy provided 
me with a couple of code snippets and explanations which helped me to 
write this stuff. It seems to work.

The start.o dependency tracking is something which didn't work in the 
old build system.