Squashed commit of the following:
configs/viewtool-stm32f107: Clean up some compilation errors. Now all builds correctly and is ready for test.
arch/arm/src/stm32: Do not condition building STM32 USB host logic on CONFIG_USBHOST. That then precludes building STM32 platforms with external USB host chips like the MAX3421E. Add an new, STM32-internal variable CONFIG_STM32_USBHOST that does the same job without such side-effects.
configs/viewtool-stm32f107: Add support for MAX3421E USB host.
Gregory Nutt