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.
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.
