genrest: List symbols selected by each symbol

Previously, you had to go to the Kconfig dump at the bottom of the
symbol reference page to see selected symbols (as opposed to the
select*ing* symbols, which were already listed). Might be easy to miss.

Use a format similar to the list of defaults
('- SELECTED_SYM if CONDITION').

Also list 'imply's, though I don't think those are used anywhere in
Zephyr yet.

Simplify the code generating the list of selecting symbols a bit as
well.

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