m68k/nommu: prevent setting ROMKERNEL when ROM is not set

When CONFIG_ROMKERNEL is set but CONFIG_ROM is not set, the linker
complains:
  m68k-linux-ld:./arch/m68k/kernel/vmlinux.lds:5: undefined symbol `CONFIG_ROMSTART' referenced in expression

# CONFIG_ROM is not set
# CONFIG_RAMKERNEL is not set
CONFIG_ROMKERNEL=y

Since ROMSTART depends on ROM, make ROMKERNEL also depend on ROM.
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Greg Ungerer <gerg@linux-m68k.org>
Cc: linux-m68k@lists.linux-m68k.org
Cc: uclinux-dev@uclinux.org
Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>

arch/m68k/Kconfig.machine

@@ -465,6 +465,7 @@ config RAMKERNEL
 
 config ROMKERNEL
 	bool "ROM"
+	depends on ROM
 	help
 	  The kernel will be resident in FLASH/ROM when running. This is
 	  often referred to as Execute-in-Place (XIP), since the kernel