makefiles: Use port.mak for level 2 and 3 as well The port.mak defines the OS level, CPU arch etc so those are not defined in the makefiles. This allows sharing of makefiles between different ports. For now the level 3 build is limited to a few disk images, and has its own makefile. At a later point we can hopefully share makefiles between level 2 and 3, checking LEVEL when needed.