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This makes building the actual object files optional to the selected
mode, which saves users who know which kind of USB mode support they
need some binary size.

Unimplemented functions are stubbed out with static inline functions.
Signed-off-by: Daniel Mack <zonque@gmail.com>
Acked-by: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: Felipe Balbi <balbi@ti.com>