Add isUARTEnable API

[Rocketct]

Implemented API for status check:

• Added API for check the UART status in Uart::flush()
• Added the API in SERCOM.h and SERCOM.cpp
  i have tested the change with the following sketch:

void setup() {
  // put your setup code here, to run once:

}

void loop() {
  Serial1.begin(115200);
  Serial1.println("asdasdasdasfasdfafasdasdas_1");
  Serial1.end();
  Serial1.println("asdasfasfasfasfafsafasdfasfsdf_2");
  Serial1.flush();
}

without the change it stop in to the first time that flush is called.

cc/ @sandeepmistry @facchinm

[sandeepmistry]

I'm going to close this for now, since we decided to take another approach inside the lib and don't need this change in the core.