defconfig: Add host mode USB tethering support for trinket

Enable USBNET config to support USB tethering in host
mode. This config enables drivers of different USB-ethernet
adapters.

Change-Id: I06be8f368690bddca572f8ec5a6820b203704f24
Signed-off-by: Sriharsha Allenki <sallenki@codeaurora.org>