makefile: Fix usage error with busybox grep
makefile: Fix usage error with busybox grep

Busybox grep does not support long options. In fact, the utilized long
options are not mandated by POSIX.1-2008. Using the short options allows
building RIOT on Alpine Linux which utilizes Busybox instead of GNU
coreutils by default.

kYc0o/RIOT,jasonatran/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,kYc0o/RIOT,kaspar030/RIOT,ant9000/RIOT,OlegHahm/RIOT,miri64/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,authmillenon/RIOT,jasonatran/RIOT,ant9000/RIOT,RIOT-OS/RIOT,miri64/RIOT,miri64/RIOT,kaspar030/RIOT,authmillenon/RIOT,kYc0o/RIOT,jasonatran/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,kYc0o/RIOT,authmillenon/RIOT,kaspar030/RIOT,OTAkeys/RIOT,ant9000/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,authmillenon/RIOT,authmillenon/RIOT,authmillenon/RIOT,miri64/RIOT,kYc0o/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,ant9000/RIOT,OTAkeys/RIOT,miri64/RIOT,ant9000/RIOT,OTAkeys/RIOT