Change the makefile so it only compiles changes.

Instead of replacing the build script with a makefile that does
essentially the same thing, have it build each object individually then
link everything together.  Since compiling on the Pi isn't terribly fast
this will save bits of time when only one object is changed.

Only tested on a Pi running raspbian, but should work as long as all
other build packages are present.

I will test cross compilation on a linux host using the cross compiler
at some point and fix if needed.