In PR #2401 hardware flow control was enabled for UART because it is recommended by Spinel specification. But pyspinel and borderrouter don't configure HW flow control by default, neither other platforms has flow control enabled by default. So, NCP setup doesn't work out of the box and required additional configuration for EFR32.

The proposal is to disable HW flow control for EFR32 for now. To follow Spinel specification recommendation it should be enabled for all platforms once pyspinel and borderrouter set HW flow control by default. Or any other complex solution. Also flow control should not be enabled for CLI application by default.