Issue #241: make Serial1 release GPIO pins on end()
Issue #241: make Serial1 release GPIO pins on end()

UARTClass::init() muxes out the UART pins to Arduino header pins 0/1.
Reset the mux on UARTClass::end() so that pins 0/1 can be used as GPIOs
again.

bigdinotech/corelibs-arduino101,sandeepmistry/corelibs-arduino101,sgbihu/corelibs-arduino101,sandeepmistry/corelibs-arduino101,linrjing/corelibs-arduino101,sgbihu/corelibs-arduino101,bigdinotech/corelibs-arduino101,yashaswini-hanji/corelibs-arduino101,facchinm/corelibs-arduino101,yashaswini-hanji/corelibs-arduino101,SidLeung/corelibs-arduino101,yashaswini-hanji/corelibs-arduino101,linrjing/corelibs-arduino101,sandeepmistry/corelibs-arduino101,SidLeung/corelibs-arduino101,eriknyquist/corelibs-arduino101,eriknyquist/corelibs-arduino101,bigdinotech/corelibs-arduino101,sgbihu/corelibs-arduino101,01org/corelibs-arduino101,facchinm/corelibs-arduino101,01org/corelibs-arduino101,01org/corelibs-arduino101