usb: Rewrote back-end allocators

Allocate back-end memory dynamically.
base-foc: Do not touch memory during map operation

Don't do anything in Mapping::prepare_map_operation.  At this point and in the
current implementation, the memory has been mapped and cleared already. Touching
the memory may only pollute the cache causing data corruption in DMA memory.

Fixes issue #452
usb: add network support for arndale board
usb grab urb sizes from driver

Instead of hardcoding the values, ask the driver for the supported sizes and
allocate the skb allocators accordingly.