Support uploading over UART1 with flasher stub (ESPTOOL-118)

[projectgus]

ESP32 supports uploading over UART1 (GPIOs 9 & 10) if a sync signal is sent to UART1 RX after booting into bootloader ROM mode. However the flasher stub is currently hardcoded to UART0.

esptool.py can still be used now if the --no-stub option is passed, but eventually the stub should be updated to support the UART1 option.

This change is also necessary to upload to ESP32D2WD over UART1, where --no-stub currently does not work.

See also forum discussion here: https://esp32.com/viewtopic.php?f=2&t=2892&p=14282#p14282

[negativekelvin]

bump

espressif/arduino-esp32#2870

[nifomorph]

Any progress on making the --no-stub work on UART1?

[andrewteta]

Is there any way to access the bootloader (using esptool.py) via UART1 on ESP32C3? If anyone knows what it would take to do this (assuming it's not a hardware limitation), could they share? I have a project that would benefit from this, so I would be willing to try and implement it.