Reboot into UART download mode without external wiring or interaction

[twischer]

This patch introduces the new method Esp.reboot_uart_dwnld().
With this method the UART download mode can be selected by software only.

When the user calls this method the ESP8266 reboots into the UART
download mode. In this mode the user can use esptool.py to flash a new
firmware file.
This patch was tested with the following command:

$ esptool.py --before no_reset --after soft_reset \
    --chip esp8266 --port /dev/ttyUSB0 --baud 460800 \
    write_flash 0x0 ./firmware.bin

The implementation is based on the original implementation in the
boot ROM. Some parts of the original implementation can be found in
https://github.com/twischer/xtensa-subjects/tree/reversed/reversed

[earlephilhower]

This PR seems to include a bunch of NONOS binaries, too. Maybe they got pulled in by accident? Can you pull them out of the PR?

Also, can you explain why this would be useful? esptool is quite capable of resetting/uploading already w/o extra code in IRAM/etc., so I don't understand the rationale.

[twischer]

Hello @earlephilhower,

This PR seems to include a bunch of NONOS binaries, too. Maybe they got pulled in by accident? Can you pull them out of the PR?

This PR is not adding new binaries. It is only modifying them:

$ git show -- tools/sdk/lib/NONOSDK*
diff --git a/tools/sdk/lib/NONOSDK221/libmain.a b/tools/sdk/lib/NONOSDK221/libmain.a
index c0e70caf..f008bc9d 100644
Binary files a/tools/sdk/lib/NONOSDK221/libmain.a and b/tools/sdk/lib/NONOSDK221/libmain.a differ
diff --git a/tools/sdk/lib/NONOSDK22x_190313/libmain.a b/tools/sdk/lib/NONOSDK22x_190313/libmain.a
index a19b330c..66403108 100644
Binary files a/tools/sdk/lib/NONOSDK22x_190313/libmain.a and b/tools/sdk/lib/NONOSDK22x_190313/libmain.a differ
diff --git a/tools/sdk/lib/NONOSDK22x_190703/libmain.a b/tools/sdk/lib/NONOSDK22x_190703/libmain.a
index 1213ceb8..cd5fd1e8 100644
Binary files a/tools/sdk/lib/NONOSDK22x_190703/libmain.a and b/tools/sdk/lib/NONOSDK22x_190703/libmain.a differ
diff --git a/tools/sdk/lib/NONOSDK22x_191024/libmain.a b/tools/sdk/lib/NONOSDK22x_191024/libmain.a
index 58779578..cd01e1a3 100644
Binary files a/tools/sdk/lib/NONOSDK22x_191024/libmain.a and b/tools/sdk/lib/NONOSDK22x_191024/libmain.a differ
diff --git a/tools/sdk/lib/NONOSDK22x_191105/libmain.a b/tools/sdk/lib/NONOSDK22x_191105/libmain.a
index 2bc98579..0f66ab6b 100644
Binary files a/tools/sdk/lib/NONOSDK22x_191105/libmain.a and b/tools/sdk/lib/NONOSDK22x_191105/libmain.a differ
diff --git a/tools/sdk/lib/NONOSDK22x_191122/libmain.a b/tools/sdk/lib/NONOSDK22x_191122/libmain.a
index 77b7de1b..260be180 100644
Binary files a/tools/sdk/lib/NONOSDK22x_191122/libmain.a and b/tools/sdk/lib/NONOSDK22x_191122/libmain.a differ
diff --git a/tools/sdk/lib/NONOSDK3V0/libmain.a b/tools/sdk/lib/NONOSDK3V0/libmain.a
index e93201c5..c30cc88a 100644
Binary files a/tools/sdk/lib/NONOSDK3V0/libmain.a and b/tools/sdk/lib/NONOSDK3V0/libmain.a differ

I have changed https://github.com/esp8266/Arduino/blob/master/tools/sdk/lib/fix_sdk_libs.sh and therefore I have applied this modification already to the libraries. This modification is required because no symbol name is set for a function I need to call.

Also, can you explain why this would be useful?

I am interested in building a quite small and chip board which is water proofed. Therefore I do not want to have any additional connectors/wiring to the outside. The only connector attached is a USB plug for power supply. But I am also connecting the UART Tx and Rx line to D+ and D- of the USB plug. Therefore UART is accessible but there is no place for any additional wiring (connected to reset and GPIO2).

In the past also others were asking for such a solution:
https://www.esp8266.com/viewtopic.php?p=38310

[earlephilhower]

Thanks for the explanation in the PR.

Overall seems like a fine idea and useful in some circumstances, but a little polish would help make it maintainable later on.

[earlephilhower]

Thanks for the add'l comments and cleanup. LGTM!

[earlephilhower]

Thanks for the cleanup and add'l comments. LGTM!

[d-a-v]

LGTM.
Regular builds are unmodified according to a quick test when this new API is not used.