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`.