USB support for PortalPlayer PP502x based DAPs

USB support is enabled as of r20105.

Minor issues

• Charging from USB is not fully implemented on some players.

logf() over USB Serial

logf() over usb serial seems to work well at full speed, not so well at high speed (but full speed should be sufficient for this)

To enable this, edit firmware/export/usb_core.h and change the following:

• remove the comment marker from #define USB_SERIAL
• (if you are only interested in logf) comment out the #define USB_STORAGE. This step is optional, but if you leave storage enabled, remember to hold the appropriate key to put the device in "charging only" mode while plugging in, or you won't be able to debug anything but storage
• You must re-run the 'tools/configure' script in your build directory and choose the '(A)dvanced' kind of build, then select '(L)ogf' to enable logf support.

You should then get a "logf over usb" in the debug menu. Use this to enable or disable the logf sending, while not connected. Results of changing this while connected are undefined.

To use this from linux, run "modprobe usbserial vendor=0x0781 product=0x7450" (for a sansa c200, change vendor and product id as appropriate, see lsusb output for yours). You should then get a /dev/ttyUSB* device when plugging in. You can then use this e.g. with "cu -l /dev/ttyUSB0", or any other serial communications program of your choice (i.e. minicom)

The usb serial implementation doesn't seem complete enough to be usable from Windows. Support under Mac OSX is unknown.