[USB Serial/JTAG] [VFS] add flag if driver has ever received bytes (IDFGH-8651)

[chipweinberger]

Related issue: #9366

This PR was spun out from #9983

introduces: usb_serial_jtag_has_driver_received_bytes();

I need a way to detect if a USB terminal is attached, so that I can decide which port to open my terminal on. If the user types a character over USB during boot, I open USB Console, otherwise I open UART Console.

(I also send a 'terminal probe request' during boot, so USB Console detection is automatic for most terminals 😊)

I like that this solution is less complex (i.e. more reliable) than calling read() or initializing linenoise or esp_console.

[CLAassistant]

All committers have signed the CLA.

[chipweinberger]

Looking around for alternatives, maybe I could instead use this function?

static inline int usb_serial_jtag_ll_rxfifo_data_available(void)

I'd just need to make sure nothing else reads the data, but I think thats already the case unless I initialize the usb_serial_jtag_driver.

[chipweinberger]

I'll test usb_serial_jtag_ll_rxfifo_data_available, and possibly close this PR if I find it to be sufficient. But, adding this flag would still be a better approach.

Edit: ah damn, just noticed its a static function. I can't call it... We should just add this flag, its so much simpler.

[chipweinberger]

Closing this PR.

This functionality can now be better implemented with bool usb_serial_jtag_is_connected(void) which was added to master on Februrary 26, 2023. 1a66459