Make NIC RX/TX buffer sizes configureable in libc

* Remove far too low default values from Nic::Connection constructor
* Extend lwip initialization function with desired TX/RX buffer sizes
* Add configuration possibility to libc_lwip_dhcp plugin to define
  buffer sizes, like the following:

  '<libc tx_buf_size="1M" tx_buf_size="1M"/>'

Fixes #892
noux: use lwip dhcp plugin, avoid code duplication

Ref #892
i.MX53 tablet: handle >1 button events concurrently

By now, only one button press/release event per IRQ was handled correctly.
Pressing and/or releasing several buttons concurrently could bring the input
driver into an inconsistent state.

Fixes #913
i.MX53 tablet: complete IOMUX routes for display

For the framebuffer driver of the i.MX53 platform to work even when u-boot
didn't prepared the display previously, there were some IOMUX routes missing.

Fixes #914
i.MX53: add config lib to GPIO driver (fix #915)
i.MX53: fit into u-boot's memory layout on tablet

Fixes #917
base/os: catch Ipc_error messages in Service class

Fixes #910
platform_drv & exynos5: provide HDMI switches

ref #796
framebuffer: support HDMI@1920x1080p on exynos5

ref #796
fb test: support exynos5

ref #796
fb test: view multiple pictures

ref #796
arndale: specs to enable demo scenario

ref #796
fb & exynos5: avoid USB HID problems with FOC

USB HID gets stuck due to strange kernel error
as HDMI starts simulatnously. This workaround
delays HDMI init by a second (only in case we
build for Exynos5 & FOC with USB) to let
scenarious like demo get their USB HID started
without much CPU load.

ref #796
demo: support exynos5

fix #796
cli_monitor: Init Ram::_resource_avail_sigh

Thanks to Stefan Kalkowski for the report.
Ring_buffer: add 'avail_capacity()' function

The 'avail_capacity()' function returns how many more elements would
currently fit into the ring buffer.

Fixes #921.
Ring_buffer: make synchronization optional

With this patch, the 'Ring_buffer' class can be made unsynchronized by
setting the 'Ring_buffer_unsynchronized' policy as third template
argument.

Fixes #922.
Noux: add basic 'Ctrl-C' support

This patch implements the POSIX signal functionality needed to interrupt a
running Noux GDB by pressing 'Ctrl-C'.

It allows to register a signal handler for the 'SIGINT' signal, which
gets executed after 'Ctrl-C' is received from the terminal. With the
current state of the implementation, the signal handler only gets executed
when the Noux application calls a 'read()', 'write()', 'ftruncate()' or
'select()' syscall.

Fixes #923.
base: 'Allocator_guard::quota' accessor
terminal_log.run: Add missing build of launchpad