After looking at mainline u-boot a bit compared to the changes that Amlogic made, it looks like it might be a better option to just rewrite what Amlogic did to fit the way standard u-boot works rather than try to just port the old code to the new release. That may be why they are still stuck on the 2015.01 codebase.
I think that the VIM will probably work with fastboot mode when updated since the Type C has otg support. However I'm not sure if the other vendors boards have otg support or not, most of the boards look like they have 3-4 regular Type A ports so I'm not sure if it would somehow work.
Ouch, thats going to take some time to go through:
1069 files changed, 257582 insertions(+), 8440 deletions(-)