With the runtime and mode portions in separate classes, a single
application should only be building with one or the other enabled. In
some applications both might be desired at build time.
The CFG_TUD_DFU_RUNTIME_AND_MODE option creates a DFU class, which asks
the application which mode to initialize to. This allows a runtime
change between RT and DFU mode, by just reinitializing tusb.
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