Coded a better payload transmit test routine for OOK

Started to write another test case to blurt out OOK energenie messages. ...

This is only a test case.
But it has highlighted a whole load of discrepancies in the original C
code to do with payload length calculation, and the 'magic' zero byte
at the start of the buffer for stuffing the SPI register address in.

Tracked down the 'long sync bit' problem. ...

It's due to the HRF always preceeding each packed with an extra 01 at start.
Existing driver code sends 'chips' to prove this.
Looking like a preamble of zeros will be required to make this a non issue,
and then generate the real 80000000 preamble as part of the payload.

hrfm69_test.c now sending a tone. ...

Discovered that 1/5 times it sticks waiting for TXREADY.
This might suggest an unresolved sequencing error in startup.

Built a simple C test harness to start testing the radio with