[U-Boot] config USB_STORAGE: defconfig vs include header

Masahiro,

In patch 6e7e9294d321 "usb: add basic USB configs in Kconfig", you added "config USB_STORAGE" to drivers/usb/Kconfig. However, it's still just #defined by many include/configs/*.h rather than being defined in configs/*_defconfig. Is that a problem? It seems to work in practice, but leads people adding new boards to put the definition in configs/*_defconfig which then may be inconsistent with similar existing boards which have it defined in include/configs/*.h.