bootstd: Drop the bootdev-specific list of bootflows

bootstd: Drop the bootdev-specific list of bootflows

This list is only used by two functions, which can be updated to iterate
through the global list. Take this approach, which allows the bootdev
list to be dropped.

Overall this makes the code slightly more complicated, but will allow
moving the bootflow list into an alist

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