Enhance idle mode

[lukasostendorf]

This PR will make the the DL channel way more silent and simplify the pilot tone generation.

• Pilot tones are now generated via an msequence that is reset at the start of every slot. This is the same for UL and DL.
• Basestation will now remain silent when there is no data to transmit
• Initial gain control had to be changed, to work with the more silent BS. We use fast_attack mode during initial gain estimation.

NOTE: This PR makes the BS and Client software incompatible with previous versions! Pilot definitions changed and the client cannot expect that the BS is always transmitting anymore.

Tests

Since we rely on manual testing, here is what tests will be performed:

• [x] Compile-able for all targets
• [x] General operability of the simulation target. Client and BS can connect and transfer data.
• [x] Verify that the BER did not increase in simulation
• [x] General operability of the standalone targets. Client and BS can connect and transfer data.

Simulations in Rayleigh fading channel for downlink show a slight performance decrease.

Simulations with static multipath in the uplink show that the BER performance is similar to the current version

[lukasostendorf]

Additionally this PR solves a conflict with the half-duplex constraint in the current HNAP version: So far, a client continuously tracks the pilot tones sent by the basestation. This also happens if the client transmits in the UL. This violates the half-duplex constraint and can cause erroneous channel estimations in the DL.