edtlib: use f-strings where it makes sense

Similarly to what was done for dtlib, use f-strings in places where it
improves readability. Some places, e.g. __repr__ methods that
construct a string using something like

    "<SomeType, {}>".format(", ".join(...))

are better left off as-is.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>