Introduce platform-specific services for core

By now all services in core where created, and registered in the generic
main routine. Although there exists already a x86-specific service (I/O ports)
there was no possibility to announce core-services for certain platforms only.
This commit introduces a hook function in the 'Platform' class, that enables
registration of platform-specific services. Moreover, the io-port service
is offered on x86 platforms only now.
base-hw: introduce VM session interface

The VM session interface is meant to be generic, but first will be used in the context
of TrustZone only.
Remove superfluous, generic Thread_state definition

The 'Cpu_state' in 'base/include/base/cpu_state.h' is not needed anymore.
Moreover, it's inconsistent with the architecture-specific definitions
of 'Cpu_state' that all reside in <cpu/cpu_state.h>.
Move away drivers from generic base-repository

Driver definitions which are used by kernel/core in base-hw, and also by other
drivers (e.g. from the os repository) have to reside in the generic
base-repository, for instance some uart drivers. All drivers which are
interesting for one of the sites only (sp804 for timer driver, or
cortex_a9 cpu driver for base-hw) should reside in the respective repos.

Factorize cpu context out of Cortex A9 specific definitions. Moreover, there
is already a Cpu_state object containing all common ARM registers. We use
this as a base for the cpu context switching done by the base-hw kernel.
The Cpu_state class get extended by a cpu-exception field, that stores the kind
of exception raised when the corresponding context got interrupted. This
information is used not only by the base-hw kernel, but also by the TrustZone
VMM that is build currently.
base-hw: increase signal session initial quota
base-hw: implement vm_session for TrustZone

* Introduces Schedule_context
* Use fast-interrupts or normal interrupts
* Add mode-transition between secure/non-secure world
* Limit system resources for Genode apps due to non-secure world

This commit implements the newly introduced Vm session interface to be used
on top of TrustZone capable Armv7 CPUs. Therefore a new Schedule_context is
introduced in the kernel. Threads and Vms are both Schedule_contexts used
by the scheduler. In contrast to a thread a vm uses a different assembler
mode switch to the non-secure, virtual world, as well as another exception
is used, when the non-secure world is left. For both worlds to co-exist
the interrupt-controller needs to be configured, so that the secure (Genode)
world uses fast-interrupts only, and the non-secure world only legacy
interrupts.
The only TrustZone capable platform the base-hw kernel works on top of
is the CoreTile Express 9x4 for the Versatile Express motherboard. For a
virtual machine working properly on top some platform resources must be
reserved. Therefore there exist two flavours of this platform now, one with
the 'trustzone' spec-variable enabled, and one without. If 'trustzone' is
specified most platform resources (DDR-RAM, and most IRQs) are reserved
for the Vm and not available to the secure Genode world.
Implement VMM for Linux in VEA9x4 normal-world
lx_hybrid: revive missing errno test