LIN Bus support, USART hardware flow control configuration support

[avtolstoy]

This PR implements #697

Hardware USART serials can be configured in one of the following LIN modes:

• LIN_MASTER_13B - LIN Master mode with ability to send 13 bit breaks
• LIN_SLAVE_10B - LIN Slave mode with 10 bit break detection
• LIN_SLAVE_11B - LIN Slave mode with 11 bit break detection

Example: Serial1.begin(9600, LIN_MASTER_13B);

These are just convenient pre-defined configurations:

// Pre-defined LIN configurations
#define LIN_MASTER_13B (uint32_t)(((uint32_t)SERIAL_8N1) | LIN_MODE_MASTER | LIN_BREAK_13B)
#define LIN_SLAVE_10B  (uint32_t)(((uint32_t)SERIAL_8N1) | LIN_MODE_SLAVE  | LIN_BREAK_10B)
#define LIN_SLAVE_11B  (uint32_t)(((uint32_t)SERIAL_8N1) | LIN_MODE_SLAVE  | LIN_BREAK_11B)

LIN break detection can also be manually enabled in Master mode (this is a supported configuration in LIN, when Master can act as a Slave as well and respond to its own messages). This configuration was used in serial_loopback test SERIAL1_LINMasterReadWriteBreakSucceedsInLoopbackWithTxRxShorted.
Serial1.begin(9600, LIN_MASTER_13B | LIN_BREAK_10B);

New Wiring methods:

• void USARTSerial::breakTx(void); - send break
• bool USARTSerial::breakRx(void); - checks if break was detected

This PR also includes support for hardware flow control settings on USARTs:

// Flow control settings
#define SERIAL_FLOW_CONTROL_NONE ((uint8_t)0b00000000)
#define SERIAL_FLOW_CONTROL_RTS ((uint8_t)0b00100000)
#define SERIAL_FLOW_CONTROL_CTS ((uint8_t)0b01000000)
#define SERIAL_FLOW_CONTROL_RTS_CTS ((uint8_t)0b01100000)

Hardware flow control is available on:

• Core: Serial1 (CTS - A0, RTS - A1)
• Photon/Electron: Serial2 (CTS - A7, RTS - RGBR (which will require some tinkering to access))

---

• Review code
• Test on device
• Add documentation (added to USART configuration fixes #997)
• Add to CHANGELOG.md

[technobly]

Nice one @avtolstoy !

[m-mcgowan]

@avtolstoy - our serial peripherals are shaping up so well! Please rebase on develop so you can add some docs to the firmware reference for the new features. Thanks!

[avtolstoy]

@m-mcgowan the docs are already in PR #997, which is based on this branch :)

[technobly]

• rebased against develop and force pushed to feature/serial-config

[technobly]

Closing this due to all of it's changes were merged into develop with #997