WARNING: If you flash a broken firmware and have no recovery mechanism, you must use the external method to flash a working firmware again.
This method does only work on Linux, if it isn't locked down. You may also need to boot with 'iomem=relaxed' in the kernel command line if CONFIG_IO_STRICT_DEVMEM is set.
For more details please also check flashrom's wiki. Use the programmer internal to flash coreboot.rom internally:
flashrom -p internal -w coreboot.rom