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.