nimble/controller: Add Channel Selection Algorithm #2 implementation
nimble/controller: Add Channel Selection Algorithm #2 implementation

This adds implementation of Channel Selection Algorithm #2 as specified
in Core Specification 5.0 Vol 6 Part B 4.6.14.

X-Original-Commit: 51d1ea26a6436be6c9fde415e0b973bc81381ce1

apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble