doc: genrest: Simplify select logic with split_expr()

The logic for walking reverse dependencies (from 'select's and 'imply's)
is a bit tricky to read (and write), and was essentially duplicated in a
few different spots (including in the upcoming menuconfig
implementation).

Use a new split_expr() helper function that's been added to Kconfiglib
to reduce code duplication and make the code more readable.

This also has another nice side effect: The list of symbols that select
a particular symbol now has the symbols in the order that they appear in
the Kconfig files.

Signed-off-by: Ulf Magnusson <ulfalizer@gmail.com>