versaloon: Finish support for serial number matching

versaloon: Finish support for serial number matching

Add a command to specify the serial string and pass it to the find helper.
Actual matching was already supported for both serial and product but was
using hard coded string indices. Instead use the indices from the device
descriptor.

Change-Id: I17c7538e5ad301ef41eace1ee70a202adab23bb4
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/1816
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>