This sample is an alternative implementation of hci_uart. The new sample
differs from the existing sample in that it uses the async UART API
instead of the interrupt driven API.
Included in this commit is a new test for HCI UART flow control. It's
enabled for hci_uart_async. The test can excercise also the existing
hci_uart sample (with some minimal changes to allow compiling in the
mock controller and test suite). The existing hci_uart sample currently
fails the flow control test.
Signed-off-by: Aleksander Wasaznik <aleksander.wasaznik@nordicsemi.no>
