make dde_linux repository an optional one

Since dde_linux now contains the port of the Linux IP stack available for all
Genode base-* platforms move the repository out of drivers_arm and drivers_x86
build.conf to the optional build.conf (available to all platforms).
netperf: Add separate run scripts for lwip/lxip

Also removed target 'app/netperf' and added 'app/netperf/lwip' and
'app/netperf/lxip'.
libports: libc_block plugin

This plugin provides support for using a Block_session connection by
opening "/dev/blkdev".

Fixes #965.
libports: add sys/disk.h symlink

Fixes #965.
Fix some typos
ram_fs: use server skeleton library

Related to #944.
base-linux: hybrid file-system server

The current implementation is missing support for symbolic links.

Fixes #944.
gpio_drv: Fix linking on PandaBoard

Issue #708
usb_drv: Use GPIO driver on PandaBoard

Also updated run scripts.

Issue #708
cpu_session: follow-up fix for scale_priority

This is a fixup for commit 5a42f40f04981581327639a75298e9e653ef39f5.

ref #961
base: Warn on undissolved signal contexts
Dissolve sig contexts in rm_fault and signal tests
hw: no default values for kernel-call args

ref #967
hw: make bootstrap save against multiple calls

In programs with dynamic linker, _main and thus also platform_main_bootstrap
are called twice. By now, platform_main_bootstrap tried to always access the
startup message in the UTCB of the main thread that gets overridden till the
second call.

fix #967
Qt5: use 'git apply' to apply patches

One of the Qt5 patches is supposed to create symbolic links, which is not
supported by GNU patch before version 2.7. Using 'git apply' to apply the
patches solves this problem.

Fixes #969.
ldso: increase MEM_SIZE in 'mmap()'

MEM_SIZE needs to get increased for the 64-bit 'linux_x86' platform when
building with -O0 on the current master branch.

Fixes #970.
pthreads: add support for recursive mutexes

Fixes #971.
Mesa: improvements for use with Qt5

Among other changes, this patch makes it possible to let Mesa render into
a user-provided buffer instead of the screen. This can be achieved with
the 'eglCreateWindowSurface()' function, which takes a buffer
description as third argument.

Fixes #974.
base-linux: round up RM session size to page size

Since RM sessions can be used as dataspaces and dataspace sizes are
supposed to have page granularity, RM session sizes should have page
granularity, too.

Fixes #972.
libc: fix size calculation in 'Mem_alloc_impl::alloc'

When allocating new backing store in 'Mem_alloc_impl::alloc()', the
requested alignment needs to be considered in the size calculation.

Fixes #973.