Normalise default serial ports for common platforms

[franky47]

Arduino / Genuino platforms currently in production

Name	Processor	SERIAL_PORT_HARDWARE_OPEN	USBCON	Definition
Uno	ATmega328P	N.A.	❌	ARDUINO_AVR_UNO
Leonardo	ATmega32U4	Serial1	✅	ARDUINO_AVR_LEONARDO
101	ARCV2EM	Serial1	❌	ARDUINO_ARC32_TOOLS
Micro	ATmega32U4	Serial1	✅	ARDUINO_AVR_MICRO
Esplora	ATmega32U4	Serial1	✅	ARDUINO_AVR_ESPLORA
Robot	ATmega32U4	N.A.	✅	ARDUINO_AVR_ROBOT_xxx
Mini	ATmega328P	N.A.	❌	ARDUINO_AVR_MINI
Mega 2560	ATmega2560	Serial1	❌	ARDUINO_AVR_MEGA2560
Zero	SAMD21G18A	Serial1	✅	ARDUINO_SAMD_ZERO
Due	SAM3X8E	Serial1	✅	ARDUINO_SAM_DUE
Mega ADK	ATmega2560	Serial1	❌	ARDUINO_AVR_ADK
Ethernet	ATmega328P	N.A.	❌	ARDUINO_AVR_ETHERNET
Yún	ATmega32U4	N.A.	✅	ARDUINO_AVR_YUN

Teensy platforms currently in production

Name	Processor	SERIAL_PORT_HARDWARE_OPEN	USBCON	Definition
Teensy 2.0	ATmega32U4	Serial1	❌	None
Teensy++ 2.0	AT90USB1286	Serial1	❌	None
Teensy LC	MKL26Z64	Serial1	❌	__MKL26Z64__
Teensy 3.2	MK20DX256	Serial1	❌	__MK20DX256__
Teensy 3.5	MK62FX512	Serial1	❌	__MK62FX512__
Teensy 3.6	MK66FX1M0	Serial1	❌	__MK66FX1M0__

Discontinued platforms

Name	Processor	SERIAL_PORT_HARDWARE_OPEN	USBCON	Definition
Pro	ATmega328P	N.A.	❌	ARDUINO_AVR_PRO
Fio	ATmega328P	N.A.	❌	ARDUINO_AVR_FIO
Nano	ATmega328P	N.A.	❌	ARDUINO_AVR_NANO
Pro Mini	ATmega328P	N.A.	❌	ARDUINO_AVR_NANO
Teensy 3.0	MK20DX128	Serial1	❌	__MK20DX128__
Teensy 3.1	MK20DX256	Serial1	❌	__MK20DX256__

[franky47]

Relates to the following issues & PR:

• #11 - Default serial port is wrong on Leonardo
• #29 - USBCON is defined only for AVR
• #33 - Digging out definitions for default serial port (lost in the middle of other topics)
• #63 - Teensy handling

[franky47]

When SERIAL_PORT_HARDWARE_OPEN is defined, it points to the hardware UART that is available for external serial IO, which is what we're after:

// These serial port names are intended to allow libraries and architecture-neutral
// sketches to automatically default to the correct port name for a particular type
// of use.  For example, a GPS module would normally connect to SERIAL_PORT_HARDWARE_OPEN,
// the first hardware serial port whose RX/TX pins are not dedicated to another use.
//
// SERIAL_PORT_MONITOR        Port which normally prints to the Arduino Serial Monitor
//
// SERIAL_PORT_USBVIRTUAL     Port which is USB virtual serial
//
// SERIAL_PORT_LINUXBRIDGE    Port which connects to a Linux system via Bridge library
//
// SERIAL_PORT_HARDWARE       Hardware serial port, physical RX & TX pins.
//
// SERIAL_PORT_HARDWARE_OPEN  Hardware serial ports which are open for use.  Their RX & TX
//                            pins are NOT connected to anything by default.

[lathoub]

Should the USB part of this table be moved over to https://github.com/lathoub/Arduino-USBMIDI?