usb: Add macro for user defined string descriptors

The new macro USBD_STRING_DESCR_USER_DEFINE works like
USBD_STRING_DESCR_DEFINE with the exception of being
ordered strictly after it. The new macro is needed to
ensures that user defined string descriptors can be added
without disturbing the order of string descriptors
defined by the usb subsystem.

Signed-off-by: Alexander Mihajlovic <a@abxy.se>