cpu/atmega_common: do not export LINKFLAGS
cpu/atmega_common: do not export LINKFLAGS

This prevent evaluating `LINKFLAGS` when not needed and when building
in docker so does not produce errors if `avr-ld` is not installed.

```
BUILD_IN_DOCKER=1 BOARD=arduino-mega2560  make --no-print-directory -C examples/hello-world/ clean
/srv/ilab-builds/workspace/git/riot_master/makefiles/toolchain/gnu.inc.mk:18: objcopy not found. Hex file will not be created.
/bin/sh: 1: avr-ld: not found
```

It removes the `/bin/sh: 1: avr-ld: not found`

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