scripts/dts: Rename convert_string_to_label() to str_to_label()

Less spammy and as easy to understand ("label" seems a bit confusing
though, but it's consistent with other code).

Also simplify some surrounding code, and clean up the implementation of
str_to_label() a bit.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>