m68knommu: fix undefined reference to `_init_sp'

m68knommu: fix undefined reference to `_init_sp'

When configuring a nommu classic m68k system enabling the uboot parameter
passing support (CONFIG_UBOOT) will produce the following compile error:

   m68k-linux-ld: arch/m68k/kernel/uboot.o: in function `process_uboot_commandline':
   uboot.c:(.init.text+0x32): undefined reference to `_init_sp'

The logic to support this option is only used on ColdFire based platforms
(in its head.S startup code). So make the selection of this option
depend on building for a ColdFire based platform.

Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>