Peer manager new dependency on nrf_atomic flags instead of atomic library.

[Susheel Nuguru]

For those who migrate to from SDKv15.0 to SDKv15.2 and still using the old project files will notice a difference that peer manager in SDKv15.2 has replaced using the below helper libraries with the ones as listed below

1. atomic flags(nrf_atflags) library instead of atomic (nrf_atomic) library. 
   
   1. Changes can be seen in peer_id.c
   2. pm_buffer,c and pm_buffer.h
2. pm_mutex library is replaced with nrf_atflags and nrf_mtx

[Susheel Nuguru]

This information slipped out of release notes in SDKv15.2.

If you get errors related to missing nrf_atflags and/or nrf_mtx then don't panic and add these source files to your projects.