Changes Makefile of ffpb-debug to make use of OpenWRT INSTALL_{DIR,

DATA, BIN} macros.

Using these macros (instead of $(CP)) in the Package/.../install section
ensures correct permissions of the files installed in the image.
The two scripts ffpb-debug and ffpb-debug now get installed using
INSTALL_BIN, while the (orphaned) cron-file gets installed using
INSTALL_DATA. In both cases we ensure that the directories exist by
calling INSTALL_DIR (with parameters "/bin" and "/lib/gluon/cron"
respectively) prior to installing the files.