sbus1_output

Two patches for sbus1_output in sbus.c.
The first implements sbus1_output for all channels except the single bit digital ones. This patch has been flown underneath ArduPlane.
The second patch cleans things up. My late night algebra was a little out with the rounding error .5f and I wasn't happy with the safety of the bounds checking. I'll hopefully get to fly this in the morning.
```
uint8_t oframe[SBUS_FRAME_SIZE] = {0};
```
Is equivalent and likely more efficient than the for loop, as the compiler can optimize it.