fsl_imx: Drop extension allowing to boot ancient i.MX CPUs via UART

[ukleinek]

The oldest i.MX CPU that I know is still in active use is i.MX25 launched in 2009. That one already uses USB for recovery booting. I think UART booting is only usable for the older i.MX1.

So I guess removing this code doesn't hurt anyone.

Compiling the source results in several warnings à la:

commands_fsl_imx.c: In function ‘fsl_sniff’:
commands_fsl_imx.c:597:17: warning: ignoring return value of ‘read’ declared with attribute ‘warn_unused_result’ [-Wunused-result]
  597 |                 read(ios->fd, &cmd, 1);
      |                 ^~~~~~~~~~~~~~~~~~~~~~

which are fixed by removing this code 😎.

[jonrebm]

@ukleinek I wanted to give it some time for objections and would like to apply now.
I think you have disabled maintainer changes on this PR so I'd kindly ask you to rebase.

[jonrebm]

On the longer run I wish we'd have some mechanism to ensure a breaking change like this makes it into the changelog. For now I guess I can just try to remember it when creating the next release.

[ukleinek]

My preferred approach here (and the one usually taken for the kernel) is to merge and let the merger resolve the conflicts (which here is just git rm commands_fsl_imx.c).
This way the commits are recorded in history as they were tested by the submitter and compared to the maintainer rebasing a topic branch (if maintainer changes are enabled) doesn't hide the responsibility.

[jonrebm]

Ok, we can do it this way.
The reason I asked was because since you've opened this PR, I've merge quite extensive CI to main (#58) and I would have liked this PR to see the new checks. But from running those checks on my rebase of this PR I can tell that it would have been green.

[jonrebm]

Applied manually in 0e63086

[jonrebm]

I realize I merged a rebased version now. This was unintentional