firmware: remove unused config.h file

Signed-off-by: Jeppe Ledet-Pedersen <jlp@satlab.org>
firmware: clean up and rename spi_tx_allow function

Signed-off-by: Jeppe Ledet-Pedersen <jlp@satlab.org>
bbctl: clean up transmit and receive functions

Signed-off-by: Jeppe Ledet-Pedersen <jlp@satlab.org>