[v2,2/2] cli: flush stdin before enabling cli

[Gregor Herburger]

Hi Simon,
> Just to check, does this option also need 'depends on !SANDBOX' like
> AUTOBOOT_FLUSH_STDIN does? That option notes that flushing stdin
> breaks autoboot unit tests. I suspect the CLI entry path might not be
> exercised by those tests, but it would be good to confirm.

I now tested a bit with the sandbox and I couldn't find any issues. I
ran some unit tests with/without this patch series and with
CMDLINE_FLUSH_STDIN enabled/disabled and couldn't find any issues. I
also ran `u-boot -c bdinfo` which seems to be executed after the
cli_init and therefore after the flush but I could't see any issues
here. 

But this is the first time for me using the sandbox so I don't know what
I could test further. If anybody has an idea what tests could be run let
me know. Otherwise I will send a v3 with an updated help text.

Best regards
Gregor