For the USB-Armory, we use a newer version of Linux (3.18) as for the
i.MX53-QSB. The main difference is, that the newer Linux uses a DTB instead of
ATAGs.
Fixes#1422
This patch changes the top-level directory layout as a preparatory
step for improving the tools for managing 3rd-party source codes.
The rationale is described in the issue referenced below.
Issue #1082