CORE: Allow using other linker than gcc

Makefile.include assumes GCC to be used for linking. Now, any board using another linker can supply LINKFLAGPREFIX = "" so that "-Wl," is not prepended to linker-specifig flags.
Please put one line of comment here like ``# We assume $(LINK) to be gcc-like. Use `LINKFLAGPREFIX :=` for ld.``