in case of CME or CMS errors, the error codes or human readable strings
are in the response buffer. We want to indicate to the AT driver user
that when tihs case occurrs so he can extract the error codes.
This defines a new 'isrpipe_read_timeout' module that should be used when using
the timeout based function of isrpipe.
This fix the implicit dependency to 'xtimer' that is only needed for the
'_timeout' functions.
It prevents 'stdio_uart' that uses 'isrpipe' to need to depend on xtimer.
This was silently solved at link time for most platforms but not for the
'esp32' for example.
'drivers/at' needed to be updated at the same time to follow the api change.
in case there's an error uart_init() returns an error value,
this value indicates the reason for the error thus we should
return it from at_dev_init() so the user will be able to identify
whether the init succeded or failed.
When reusing the same buffer for the at command and response, no command
would be sent because the buffer was cleared. This is fixed by only
clearing the buffer after the command has been sent.
