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.

Only 100-200 bytes more flash usage.