cpu/kinetis/check-fcfield: use OBJDUMP
cpu/kinetis/check-fcfield: use OBJDUMP

Get OBJDUMP from the environment instead of hardwriting the value.

This is a prerequisite to allow using `objdump` when building from docker
when not having the `arm` toolchain installed.

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