[realtek-ambz2] Fix C++ support, implement SerialClass

docs/contrib/porting.md

@@ -50,6 +50,7 @@ Here's what has to be done to make that work:
 	- Make sure not to make a mess in the `CCFLAGS`/`CPPDEFINES`, and only include what's needed there. Some flags are project-wide (family-independent) in `builder/frameworks/base.py`.
 	- Use a **pure PlatformIO** project - **not ESPHome!**. Pass one of the generic boards you created before, and `framework = base` in `platformio.ini`. Generally, try to get the thing to compile.
 	- Use a simple Hello World program - C, not C++. Only add `main()` function with a `printf()` and a `while(1)` loop.
+	- I've noticed that using `nano.specs` instead of `nosys.specs` produces smaller binaries.
 
 9. When you get it to link successfully, build a UF2 file.
 
@@ -91,3 +92,9 @@ Here's what has to be done to make that work:
 5. Write LibreTiny C APIs - in `lt_api.c`.
 
 6. At this point, your Hello World code should work fine.
+
+## Porting Arduino Core - C++ support
+
+1. Add main.cpp and write wiring_*.c ports. GPIOs and stuff should work even without proper C++ support.
+
+2. Port Serial library first. This should already show whether C++ works fine or if it doesn't. For example, calling `Serial.println()` refers to the virtual function `Print::write`, which will probably crash the chip if C++ is not being linked properly.

docs/flashing/tools/adr.md

@@ -16,3 +16,7 @@ The code listens on UART1 for a link-check command (`01 E0 FC 01 00`). The baudr
 ## Realtek AmebaZ
 
 This only works when using [ltchiptool](ltchiptool.md) for flashing. Upon starting UART communication, the tool sends `55 AA 22 E0 D6 FC` (0x55AA followed by the `realtek-ambz` family ID). After detecting that pattern, the chip proceeds to reboot into UART download mode (using [`lt_reboot_download_mode()`](../../../ltapi/lt__device_8h.md))
+
+## Realtek AmebaZ2
+
+The code listens on UART2 for a `ping\n` command, that is sent by [ltchiptool](ltchiptool.md) (and possibly by the vendor flasher, too). The device is then rebooted to download mode.