makefiles/vars.inc.mk: do not export LINKFLAGS
makefiles/vars.inc.mk: do not export LINKFLAGS

It is only used by `Makefile.include` and should not be used by sub-make
instances.

This is required to prevent evaluating `LINKFLAGS` when not needed and a
required step to not evaluate it on the host with for example `avr-ld`
when building in docker.

I checked usage with:

    git grep -e '(LINKFLAGS)' -e '{LINKFLAGS}'

Packages may be using it but `LINKFLAGS` is a RIOT way of naming
things and even if `generate-xcompile-toolchain` uses `LINK` it does not
use `LINKFLAGS`.

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