new_variant: docs cleanup

Minor clean-up:
* use "Reference Board" more consistently
* add some explanatory text to the tutorial intro
* shorten URLs for gerrit
* clarify the origin of the coreboot upstream CL

BUG=None
TEST=N/A

Change-Id: I84cc7b4f3796fd25be79148dca8f5d916638a07b
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/2269948
Tested-by: Paul Fagerburg <pfagerburg@chromium.org>
Reviewed-by: Jack Rosenthal <jrosenth@chromium.org>
Commit-Queue: Paul Fagerburg <pfagerburg@chromium.org>
1 file changed