If USB CDC is connected and the board sends data, but the host does not
receive the data, the device locks up. This is fixed in this commit by
having a timeout of 500ms, after which time the transmission is skipped.
Following on from ba940250a5, the change here
makes output about 15 times faster (now up to about 550 kbytes/sec).
tinyusb_cdcacm_write_queue will return the number of bytes written, so
there's no need to use tud_cdc_n_write_available.
Signed-off-by: Damien George <damien@micropython.org>
This callback allows detecting if there is a USB host connected to the CDC
or not, in which case the stdout_tx should skip CDC TX writing and
flushing or the system will block.
Fixes issue #7820.
