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`
9cd50a693a