New DDE-Linux-based USB driver

The new 'dde_linux' repository will host device drivers ported from the
Linux kernel. In contrast to the original 'linux_drivers' repository,
'dde_linux' does not contain any 3rd-party source code. To download the
Linux kernel source code and extract the drivers, execute the 'make
prepare' rule of the top-level Makefile. The initial version of the
'dde_linux' repository comes with an USB driver. The porting methodology
follows the path of the Intel GEM port. Instead of attempting to provide
a generic Linux environment that works across drivers, each driver comes
with a specially tailored DDE.

The DDE consists of Genode-specific implementations of Linux API
functions as declared in 'lx_emul.h'. Most of these functions are
dummies that must merely be provided to resolve dependencies at the
linking stage. They are called by unused code-paths.

As of now, the USB driver support UHCI, EHCI on the x86_32 platform. I
exposes USB HID devices and USB storage devices via Genode's input-session
and block-session respectively.

The USB driver is accompanied with two run scripts 'run/usb_hid.run' and
'run/usb_storage.run'.

.gitignore

@@ -13,6 +13,8 @@
 /base-okl4/download
 /base-pistachio/contrib
 /dde_ipxe/contrib
+/dde_linux/contrib
+/dde_linux/download
 /libports/contrib
 /libports/download
 /libports/include/EGL/egl.h