Define optimisation

Hello,

I'm actually trying to clean up Marlin a bit, therefore i have moved some of the preprocessor defines around.
And added some where I guessed they would be useful.
Maybe someone is willed to help me a bit. 

So what are my goals with this:
 - saving space and memory on the processor
 - reducing calculation times through deleting unnecessary codes (if not already done by the compiler)
 - removing compiler warnings

Greetings
Salandora