m68knommu: clean up linker script

There is a lot of years of collected cruft in the m68knommu linker script.
Clean it all up and use the well defined linker script support macros.

Support is maintained for building both ROM/FLASH based and RAM based setups.
No major changes to section layouts, though the rodata section is now lumped
in with the read/write data section.

Signed-off-by: Greg Ungerer <gerg@uclinux.org>