Changes Makefile of ffpb-alfred-watchdog 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 script
ffpb-alfred-watchdog itself now get installed to /bin using INSTALL_BIN,
while the corresponding 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.