make/docker: adapt mount of localtime
make/docker: adapt mount of localtime

Mounting `/etc/localtime` directly does not work on macOS (anymore).
However, by resolving the symlink to its real path docker can handle
the mount.

jasonatran/RIOT,rfuentess/RIOT,miri64/RIOT,OTAkeys/RIOT,authmillenon/RIOT,rfuentess/RIOT,A-Paul/RIOT,OTAkeys/RIOT,ant9000/RIOT,OlegHahm/RIOT,authmillenon/RIOT,OlegHahm/RIOT,authmillenon/RIOT,OlegHahm/RIOT,aeneby/RIOT,lazytech-org/RIOT,josephnoir/RIOT,lazytech-org/RIOT,mtausig/RIOT,authmillenon/RIOT,jasonatran/RIOT,ant9000/RIOT,authmillenon/RIOT,cladmi/RIOT,jasonatran/RIOT,toonst/RIOT,smlng/RIOT,OTAkeys/RIOT,toonst/RIOT,gebart/RIOT,josephnoir/RIOT,toonst/RIOT,rfuentess/RIOT,smlng/RIOT,miri64/RIOT,miri64/RIOT,kYc0o/RIOT,BytesGalore/RIOT,mfrey/RIOT,ant9000/RIOT,BytesGalore/RIOT,lazytech-org/RIOT,aeneby/RIOT,yogo1212/RIOT,OTAkeys/RIOT,cladmi/RIOT,RIOT-OS/RIOT,cladmi/RIOT,mtausig/RIOT,kaspar030/RIOT,kaspar030/RIOT,x3ro/RIOT,kYc0o/RIOT,yogo1212/RIOT,lazytech-org/RIOT,kYc0o/RIOT,A-Paul/RIOT,x3ro/RIOT,A-Paul/RIOT,cladmi/RIOT,gebart/RIOT,authmillenon/RIOT,smlng/RIOT,kYc0o/RIOT,yogo1212/RIOT,basilfx/RIOT,basilfx/RIOT,OlegHahm/RIOT,BytesGalore/RIOT,x3ro/RIOT,mtausig/RIOT,kYc0o/RIOT,aeneby/RIOT,kaspar030/RIOT,aeneby/RIOT,cladmi/RIOT,ant9000/RIOT,gebart/RIOT,lazytech-org/RIOT,BytesGalore/RIOT,RIOT-OS/RIOT,toonst/RIOT,rfuentess/RIOT,OTAkeys/RIOT,yogo1212/RIOT,toonst/RIOT,basilfx/RIOT,miri64/RIOT,A-Paul/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,gebart/RIOT,basilfx/RIOT,mfrey/RIOT,A-Paul/RIOT,yogo1212/RIOT,gebart/RIOT,x3ro/RIOT,kaspar030/RIOT,ant9000/RIOT,smlng/RIOT,jasonatran/RIOT,x3ro/RIOT,mfrey/RIOT,mtausig/RIOT,rfuentess/RIOT,RIOT-OS/RIOT,mfrey/RIOT,basilfx/RIOT,kaspar030/RIOT,yogo1212/RIOT,mtausig/RIOT,OlegHahm/RIOT,josephnoir/RIOT,RIOT-OS/RIOT,josephnoir/RIOT,aeneby/RIOT,BytesGalore/RIOT,mfrey/RIOT,miri64/RIOT,josephnoir/RIOT,smlng/RIOT