bootstd: Move label parsing into its own function

bootstd: Move label parsing into its own function

This is complicated enough to merit its own function, particularly as we
are about to add to it. Create a new label_to_uclass() function to decode
a label.

Also update the code to ignore an empty label or one consisting of just a
number.

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