SWSerial wasn't even building due to a typo in the header, and SerialPIO
needs to set the OE-invert flag after PIO initialization for transmit.
Fixes#2419
Use real GPIO pad inversion to allow inverted RX, TX, and controls for
the hardware UART and software PIO-emulated serial ports.
Adds ``setInvertTX(bool)`` and ``setInvertRX(bool)`` calls to both ports,
with ``setInvertControl(bool)`` for the HW UARTS.
I receive mails weekly asking how to use `SoftwareSerial` on this core.
Avoid the issue by including a simple wrapper class around `SerialPIO`
which gives the proper class name and constructor parameters.
Note that inverted mode is not supported.
