CMSIS-DAP: auto-detect CMSIS-DAP USB VID:PID

CMSIS-DAP: auto-detect CMSIS-DAP USB VID:PID

The current OpenOCD implementation requires CMSIS-DAP adapter USB
VID:PID values to either be hard-coded in the source or manually
supplied by the user's configuration files.  The CMSIS-DAP
specification stipulates that all compliant adapters should have
"CMSIS-DAP" in the product string.  This should obviate the need
for hard-coding.

This patch was previously submitted as changes 1882, 1883, and 1886
but amendments failed to be registered by the server.  The
functionality was changed from 1886 in response to comments so that
user-supplied VID:PID values overrided the CMSIS-DAP auto-detect.

Change-Id: Ifb2dc217248359f448e0a42bd1527dd744c434b0
Signed-off-by: Peter Lawrence <majbthrd@gmail.com>
Reviewed-on: http://openocd.zylin.com/1888
Tested-by: jenkins
Reviewed-by: Xiaofan <xiaofanc@gmail.com>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>