Portable has two meanings: a construct is portable if we can rely on it to work everywhere. A portable codebase is one that supports moving to a new platform, with some nonzero effort which is not large compared to rewriting the code. It is able to be ported. E.g, "Johnson's Portable C Compiler (PCC)".
If only the Foo kernel must be rewritten in order to port Foo, but that kernel is 75% of Foo, then I would say Foo is not portable. If the kernel is 0.1% of Foo, then I would say that it is: 99.9% of the code base depends on the abstractions in the Foo kernel rather than platform features.