bootstd: Allow iterating to the next label in a list

bootstd: Allow iterating to the next label in a list

Add a function which moves to the next label in a list of labels. This
allows processing the boot_targets environment variable.

This works using a new label list in the bootflow iterator. The logic to
set this up is included in a subsequent commit.

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