Move radio-agnostic code to an external shared file

[avsa242]

It's cumbersome to maintain and modify the main application when the effort must be duplicated across each variant (e.g., CC1101, CC2500, SX1231).

• Move any code that isn't model-specific to a file that can be shared

Move radio model-specific code to new top-level files (these would be the apps that are compiled/loaded)

• CC1101
• CC2500
• SX1231

[avsa242]

Initial rewrite in 0396dcd
Port cc1101-specific app

[avsa242]

Port cc2500-specific app in 2e3e7d0

[avsa242]

Port sx1231-specific app in 233261a (rssi capture isn't working yet)