[diag] quick fix to support long output (#3436)
[diag] quick fix to support long output (#3436)

`Cli::Uart::OutputFormat()` current only supports output
`OPENTHREAD_CONFIG_CLI_MAX_LINE_LENGTH` bytes, which isn't enough
for command *diag* by default. This PR removes that restriction
by using `Cli::Uart::Output`.

I think this is just a quick fix, an overall enhancement of Cli is
needed so that we can use small stack size but also get better
performance.
chshu/openthread,erja-gp/openthread,erja-gp/openthread,turon/openthread,srickardti/openthread,chshu/openthread,turon/openthread,turon/openthread,turon/openthread,chshu/openthread,lanyuwen/openthread,jwhui/openthread,erja-gp/openthread,lanyuwen/openthread,lanyuwen/openthread,librasungirl/openthread,librasungirl/openthread,erja-gp/openthread,openthread/openthread,lanyuwen/openthread,bukepo/openthread,openthread/openthread,abtink/openthread,erja-gp/openthread,abtink/openthread,bukepo/openthread,srickardti/openthread,bukepo/openthread,openthread/openthread,chshu/openthread,chshu/openthread,srickardti/openthread,srickardti/openthread,openthread/openthread,librasungirl/openthread,chshu/openthread,abtink/openthread,erja-gp/openthread,jwhui/openthread,bukepo/openthread,librasungirl/openthread,jwhui/openthread,jwhui/openthread,lanyuwen/openthread,abtink/openthread,lanyuwen/openthread