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>