base-files: cleanup led functions in uci-defaults.sh

create a function with code common to all led functions,
create another function with code common to functions setting
a simple led trigger, restore alphabetical order in function names.

Signed-off-by: Alberto Bursi <alberto.bursi@outlook.it>
[Jo-Philipp Wich: use a single underscore to denote private functions]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>