Refactor 4450/4551 to use common code

[terrillmoore]

Many routines like GetSysEUI() are duplicated in the samd and stm32 dirs because of different FRAM types. But the FRAMs (2k and 8k) export a common API so instead of this->m_Fram we could use this->GetFram() as long as GetFram() returns an Fram reference (Fram&).

For others (like registerCommands) there seems to be no good reason for them to be separate (though again it may be the FRAM stuff.

[terrillmoore]

@chwon64, did you see this? This idea would be a global substitution to change every this->m_Fram reference to this->GetFram() (and implement that method). Since this could return an Fram&, we could get refactor the rest of the code that is dependent on the FRAM type.