m68k: generalize uboot command line support

The uboot command line support needs to be used by both MMU and no-MMU
setups, but currently we only have the code in the no-MMU code paths.
Move the uboot command line processing code into its own file. Add
appropriate calls to it from both the MMU and no-MMU arch setup code.

Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>