Revise platform-specific driver detection about printipi

[Wallacoloo]

This is now implemented in devel.

However, it may also be desirable for the makesystem to pick up all *.cpp files in the drivers/[machine]/ directory and compile those too, because the dynamic detection only really works for the header files.

It can either pick up all *.cpp files, *.cpp files matching known driver interfaces, or call a recursive Makefile (though those are not friendly to work with in my experience).

[Wallacoloo]

Makefile in devel now picks up the driver-specific *.cpp files. Haven't tested to see if it's recursive (e.g. drivers/[platform]/private-dir/x.cpp).