Add support for custom peripherals

[giomba]

This branch implements a dynamic bus module, which can be used to register custom peripherals to the computer's bus.
The idea is that the standard configuration of the computer is statically described in the bus array in bus.c.
When a peripheral is not found in the main bus, then it is searched via the ubus.c module.
The ubus.c module contains a small array where an user can register its own peripherals at runtime, by calling ubus_register() and providing the necessary address range and callbacks for I/O.
(Obviously this is not how it works on the real hardware, but it looks a reasonable way to implement user-built peripherals in the emulator, by separating the vanilla bus from the customized one)

Then, this branch also implements a charmon.c fake write-only character based device: at the moment, it can be used by the emulated software to log data on the emulator's stdout, and also acts as an example of how to properly implement a custom peripheral module. (note the dynamic configuration via the user's conf file)

Eventually, the ubus.c module can also be used to implement the actual hardware mods that we are planning.

[giuliof]

Approved, just added a note related to logging

[giuliof]

Added one more note