Pod program cannot be run without cross-compilation

[taesungh]

Following from #1/#20 and #7/#23, we decided on using RPPAL for peripheral control and providing ways to cross-compile for the Raspberry Pi's architecture. However, RPPAL will not compile on operating systems other than Linux, and will generally be unusable (e.g. UnknownModel error) when not running on a Raspberry Pi.

To allow testing other non-peripheral components of the pod-operation program such as the FSM, we'll need to incorporate some form of mocking and conditional compilation. One possible option is using dbrgn/embedded-hal-mock and using the cfg macro to substitute for RPPAL when not building for the Raspberry Pi.

[taesungh]

Reprioritizing this to make it easier to test the GUI with the FSM without needing to disable all of the RPPAL components. embedded-hal-mock places some constraints on expectations of values which is a little too strict for our purposes. We might instead consider just adding our own mock implementations of the higher-level component methods which provide placeholder values without interfacing through RPPAL.