Don't hard-code for ncc in mig and ncg scripts.

The rationalization for this change is that nesc shouldn't really know where
tinyos-tools is installed, since they are separate packages.  The pragmatic
reason for this change is that it allows using ncc from tinyos-tools before
installation to allow a single debian tinyos source package to build both
tinyos-tools and tinyos-source-VERSION packages.