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>
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 8010afa..1df1326 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -69,7 +69,7 @@
       fi
     - if [[ "${TEST_PY_BD}" == "coreboot" ]]; then
         wget -O -
-          "https://drive.google.com/uc?id=1x6nrtWIyIRPLS2cQBwYTnT2TbOI8UjmM&export=download" |
+          "https://drive.google.com/uc?id=1uJ2VkUQ8czWFZmhJQ90Tp8V_zrJ6BrBH&export=download" |
           xz -dc >${UBOOT_TRAVIS_BUILD_DIR}/coreboot.rom;
         wget -O -
           "https://drive.google.com/uc?id=149Cz-5SZXHNKpi9xg6R_5XITWohu348y&export=download" >