Add conditional compilation flags to reduce compiled binary size.

[SFE-Brudnerd]

Problem Statement

Currently, this framework is all or nothing, which can bloat the binary. While this is acceptable for the datalogger product it's currently used in, it makes it difficult to utilize this in future products that may not want to use all the sensors, only some of them.

Proposed Solution

Using some sort of conditional compilation. 2 ideas come to mind to me.

1. #ifdef guarding inside the device libraries with a configuration file in your application including the appropriate devices. This could take the form of #ifdef (FLX_DEV_ALL || FLX_DEV_ABC123) so there's still an include all option.
2. Using CMake. I'm not sure how this would work in the context of arduino applications, but arduino cores use it. Would require further thought.

[gigapod]

This was done for the IoT LoRaWAN work, but not in the main line of the framework.

A CMake system is in place to build a cutomizable Arduino Library that can be used by a desired application