Add CMSIS-DAP v2 support

This change implements CMSIS-DAP v2 which works with raw USB bulk transfers.

The old driver is now split into a generic CMSIS part and a HID backend,
with a new raw USB backend for CMSIS-DAP v2.

New commands:
- cmsis_dap_backend (usb_bulk | hid | auto)
- cmsis_dap_usb interface <interface number>

Change-Id: I4218477b12ccbfe19c9b332321cd21394bf44e30
Signed-off-by: Mickaël Thomas <mickael9@gmail.com>
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/4831
Tested-by: jenkins