Add flow control

[nseidle]

This PR adds flow control support for all UARTs (0 and 1).

It was fairly straight forward. Below is the test sketch that shows CTS properly working. I haven't devised a succinct way to test RTS (output).

Uart SerialTest(1, 25, 12, 10, 17); //Instance, RX, TX, RTS, CTS

void setup()
{
SerialTest.begin(115200);
SerialTest.println("SerialTest online");
}

void loop()
{
SerialTest.println("Stop this text by pulling CTS(17) high");
}

[oclyke]

Cool! My rough plan is to do a final release of v1.X.X before we release v2.0.0. I will include this in that (after reviewing it too).

All the open PRs are not in vain - at the very least they can help us implement the same features in mbed and v2.0.0

[oclyke]

Looks good - I will merge. However please see my comment about changing the HAL. We can use our SDK mirror repo to better document and distribute fixes among our codebase. I have discussed this repo with Arpit and Ron at Ambiq and they are interested in using it to help discover / report bugs.