USB: provide low-level internal TX and RX buffers

USB: provide low-level internal TX and RX buffers

Some USB controllers may require specific constraints on the buffers
used for the endpoint transfers (e.g., a proper memory alignment).

Add CONFIG_USB_RXBUFSIZE and CONFIG_USB_TXBUFSIZE to the usb module
configuration parameters to define the size of these buffers.

If a low-level driver has specific memory alignment constraints it can
define such buffers internally using the proper alignment.

git-svn-id: https://src.develer.com/svnoss/bertos/trunk@4342 38d2e660-2303-0410-9eaa-f027e97ec537