If only buffer-full events and LF characters trigger the flush of
the send buffer then only line-based communication can be
implemented over the BLE UART Service. This patch extends the
service API by adding an explicit `flush` method to force sending
the buffer contents, thus enabling protocols with short (e.g.,
single character) messages.
In the process, the path has been shortened and the unecessary ble
directory between FEATURE_BLE and the actual implementation has been
removed.
commit id of ARMmbed : 17728a824c7273e16b6b74fae871e2997c88ecd1