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>

arch/m68k/Kconfig.machine

@@ -335,6 +335,7 @@ comment "Machine Options"
 
 config UBOOT
 	bool "Support for U-Boot command line parameters"
+	depends on COLDFIRE
 	help
 	  If you say Y here kernel will try to collect command
 	  line parameters from the initial u-boot stack.