CI: x86: coreboot: Update to latest coreboot

Use a recent coreboot build for this test.

The coreboot commit is:

   6f5ead14b4 mb/google/nissa/var/joxer: Update eMMC DLL settings

This is build with default settings, i.e. QEMU x86 i440fx/piix4

Add some documentation as to how to update it next time.

Signed-off-by: Simon Glass <sjg@chromium.org>