cmd: exit: Doesn't function

[Adrian Vovk]

Hello,

I've run into an issue trying to use a recent commit of u-boot
(v2022.07-rc3-101-g90189ecd59) with some custom scripts. Basically, I've
found that the `exit` command does not function whatsoever, and instead it
just behaves as a no-op (like the `true` command)

I tracked down the issue and found its source. It's 8c4e3b7[1], which
changed the behavior of the exit command without changing the behavior of
the command processor to match. The command line code expects exit to
return values -2 and lower, which it will respond to by exiting[2].
However, with that commit above, exit will never return a value <= -2,
which means the command line loop never exits.

I skimmed the mailing list and haven't found any mentions of this bug yet.
I suspect this is because many of the deployments of u-boot are running
older versions. However, exit is used rather often in the u-boot scripts
I've seen, so I think this might become a major issue when board vendors
start upgrading to newer versions of u-boot.

[1]:
https://source.denx.de/u-boot/u-boot/-/commit/8c4e3b79bd0bb76eea16869e9666e19047c0d005
[2]:
https://source.denx.de/u-boot/u-boot/-/blob/master/common/cli_hush.c#L1901-1904

Thank you,
Adrian Vovk