hw_rpi: Hand out IRQ for USB host controller
dde_linux: Follow symlinks while grepping includes

This patch principally allows to install symlinks to out-of-Linux tree
drivers into the contrib directory. Those files are then considered for
the 'lx_emul.h' symlink procedure. Is useful as a temporary mechanism
while developing the rpi USB driver.
dde_linux: Limit number of merged IRQs

This change makes Irq_context::_handle_one robust against misbehaving
IRQ handlers that always return IRQ_HANDLED, specifically dwc_otg.
dde_linux: Support for Raspberry Pi

At the current stage, the USB HID and storage drivers are prinicpally
working but not stable. If interrupts are not processed fast enough,
devices will get sporadically disconnected.

The USB host-controller driver is not part of the normal Linux kernel.
For this reason, we need to download it separately. There exists a
'prepare_rpi' rule in the 'dde_linux/Makefile' to automate this process.
rpi: Videocore cache definitions
dde_linux: USB networking for rpi

Even though the network adaptor gets initialized, networking is not
functional yet.
rpi: Change link address to support larger images
rpi: Allow the access to videocore memory as MMIO
rpi: Framebuffer and platform drivers

The platform driver is used to access the features provided by the
Videocore mboxes, i.e., power configuration and framebuffer setup. The
framebuffer driver uses the platform interface to setup a screen mode of
1024x768.
dde_linux/rpi: Enable USB power on startup

Needed when booting Genode directly from SD card w/o u-boot.
Do not try to start Qemu for platform_rpi
demo.run: Support for Raspberry Pi