Add begin write and end write hints. If implemented by the serial
driver, then the driver can buffer up data sent via serialWrite() and
flush it when serialEndWrite() is called.
Implemented at the buffer level as it requires the least change to how
serial_msp and serial_cli are architected.
Also tidy up the visibility in the VCP driver.
Signed-off-by: Michael Hope <mlhx@google.com>
This lets USB send up to 32 bytes in a frame instead of 32 one byte
frames. Add a fallback for drivers that don't implement writeBuf().
Signed-off-by: Michael Hope <mlhx@google.com>
