Massively speed up U-Boot parsing

U-Boot has a ton of definition locations for some symbols, causing a lot
of redundant work when iterating over Kconfig.defined_syms in
_build_dep(). Iterate over set(Kconfig.defined_syms) instead, wherever
possible.

This speeds up the U-Boot parsing time from 4 seconds to 0.6 seconds on
my machine.

Also update the bundled tools to iterate over set(Kconfig.defined_syms).
The performance loss is negligible even for projects that don't use
multiple definition locations.

Update the documentation to clarify that symbols/choices defined in
multiple locations appear multiple times in Kconfig.defined_syms/choices
as well.

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