Add picoprobe-only EXTRA_UART cmake option

[milipropasm]

bridges 2nd hardware UART over USB-CDC as tty1 New CDC_UARTS count placed in CMakeLists.txt to ensure tusb_config.h and cdc_uart.c can see it.

CMakeLists.txt, picoprobe_config.h, board_pico_config.h, tusb_config.h, main.c, cdc_uart.h, cdc_uart.c, usb_descriptors.c Extra usb descriptor, interfaces and endpoints when needed. desc_ms_os_20 NOT updated. cdc_task() gets new tty index param (extant UART always tty0). was_connected and usb rx/tx buffers now module-static arrays with tty index. All tud_cdc_* calls changed to tud_cdc_n_* variants, PICOPROBE_UART_RX/TX_LED remain tty0-only. Basic testing: Pi4B host, THREADED 1, DAP_V2: minicom receives uart output(s), openocd 'program xxx.elf verify reset exit' works.

[andrethomas]

Thanks, this is very useful

[hwmland]

Any chance that this PR will be merged any time soon?

[P33M]

Please retest this on top of latest master - there have been changes which will impact this PR.