microbit-dal: serial SYNC_SLEEP / SYNC_SPINWAIT modes transmit all characters [issue #141]
microbit-dal: serial SYNC_SLEEP / SYNC_SPINWAIT modes transmit all characters [issue #141]

In SYNC_SLEEP and SYNC_SPINWAIT modes the maximum length that could be
sent was always dictated by the buffer size.

This was incorrect as these two modes should wait until all characters
have been transmitted from the given buffer before returning control to
the user.

ASYNC mode will still return immediately after copying the number of
bytes available in the TX buffer.

markshannon/microbit-dal,markshannon/microbit-dal