SoftwareSerial implementation

[ABOSTM]

Summary

This Pull Request is based on PR: Add SoftwareSerial #627 from @armint:
Add SoftwareSerial for STM32 boards.

This PR adds SoftwareSerial support for STM32 boards. The code was based on the SoftwareSerial implementation from the lpc176x arduino port and adapted for STM32. It uses the recently added HardwareTimer code for timing. The timer to be used can be defined in variant.h by defining TIMER_SERIAL. When no TIMER_SERIAL is defined it will use the simplest timer available.
Tested and working on STM32F446ZE and STM32F401RE. Works with existing SoftwareSerial example code. Tested at 9600 and 115200 baud by communicating with hardware serial interfaces and by logic analyser.

Besides PR #627, I also

• remove the actual oversampling because I already have issue with baudrate 115200 (on NUCLEO_STM32L476RG @ 80MHz) because I spend 100% CPU load in intterrup, waiting for some RX. And thus unable to handle TX even when nothing is received. And actual oversampling would increase Interrupt overhead.
• keep original (lpc176x) OVERSAMPLING management to sample in RX in the middle of the bit.
• add some comment to explain the management of bits/tick

Tested on NUCLEO_STM32L476RG: up to Baudrate 57600 test passed. Failed at 115200(too much time spend in IT).
Both examples working:

• SoftwareSerialExample
• TwoPortReceive

Fixes #267

[fpistm]

See ABOSTM#2