CDC Tx No Overrun

[alex-dewar]

CDC serial TX can easily overrun the onboard buffer, as it doesn't check if there's any space before writing to it. Likely causes serial TX issues in the case where large amounts of TX data is sent (i.e. in the case of a serial bootloader)

Unclear the best way to actually fix this - there's a UDI_CDC_TX_EMPTY_NOTIFY() define that seems promising, but it's unclear what it actually does.