Change in flashrom[master]: Makefile: build shared usb code only when needed by programmer

[Nico Huber (Code Review)]

Attention is currently required from: Thomas Heijligen, Angel Pons.
Nico Huber has posted comments on this change. ( https://review.coreboot.org/c/flashrom/+/62832 )

Change subject: Makefile: build shared usb code only when needed by programmer
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Patch Set 1:

(1 comment)

Patchset:

PS1:
> This makes it clear which files are needed for the individual programmer. […]

Yes, that is correct. I just want to make sure we are not too pedantic
for our own good.

Even if it's not 100% correctly modelling reality, the current state seems
easy enough to maintain. Unless I miss something, e.g. do we need to know
what files are needed for each programmer? does that help us with some-
thing?

I'm a bit concerned because in the past there were too many things to take
care of to add a new programmer. So I'd like to make it as easy as possible
as long as it doesn't cause problems somewhere else.