Add helper for splitting expressions

I've had to implement the logic for walking reverse dependencies (from
select) a couple of times now, and it's always a bit tricky to get
right. Reduce code duplication and simplify things by adding a helper
function split_expr() that splits an expression into a list of either
its AND or OR operands.

A nice side effect is that e.g. the warning generated for selecting a
symbol with unsatisfied direct dependencies now lists the selecting
symbols in the order that they appear in the Kconfig files.

split_expr() might be helpful for splitting other types of expressions
as well, e.g. to put operands on separate lines when generating
documentation.

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