There are eight key files in the common folder. The files are converted
to header files on compile time and the array of the keys is filled with
them. After determining the device id via the pin programming the
corresponding key is chosen.

Each of the keys is
mapped to one station id.

On start-up the sensor sends a Get_Salt-message to the base station. The
base station responds and sends the salt, which the sensor uses from
there on.

The nrf24l01 used on the boards is probably a counterfeit chip. It
cannot acknowledge received packets (probably a bit flip of the NO_ACK
bit described here:
https://ncrmnt.org/2015/03/13/how-do-i-cost-optimize-nrf24l01/) if the
dynamic packet length is disabled.

Without it the bits are sometimes read incorrectly.

The address was set for the wrong address and thus the module could not
receive acks from the base station

The packet length is now fixed to 32 bytes.

The power cycling needs to be performed with CS high.

Improves compatibility with counterfeit "NRF24L01" chips.

Sending still uses busy waiting, using interrupts there requires some
additional restructuring.

Improves compatibility with counterfeit "NRF24L01" chips.