makefiles: Use `grep -q` rather than `--quiet`
makefiles: Use `grep -q` rather than `--quiet`

`--quiet` is a GNUism; Alpine's busybox grep only supports the minimal
-q option required by POSIX.

kaspar030/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,kaspar030/RIOT,kaspar030/RIOT,kaspar030/RIOT,RIOT-OS/RIOT