Don't require an IDE preferences file, fallback to default sketchbook.

It should be possible to use the Makefile without ever running the
Arduino or chipKIT IDE. This change only looks for the preferences file
if the sketchbook path is not defined, and even then, will use the
default skethbook path if the preferences file is unavailable.
Merge pull request #108 from openxc/no-preferences-required

Don't require an IDE preferences file, fallback to default sketchbook.